Update, August 26, 2015: I’ve updated this article so that its code works with Swift 2. It compiles under the latest version of Xcode 7, beta 6.
So far in this series, we’ve looked at:
 The basics of dates and times in Swift, with creating dates, converting components into dates and vice versa, and converting dates into strings and vice versa, and
 date arithmetic in Swift, with adding time intervals to dates and calculating the time between two dates.
So far, everything we’ve done has a distinctly unSwiftlike feel to it. That’s because Cocoa’s date and time classes were built with its original programming language, ObjectiveC, in mind. In this article, we’ll look at ways to make date calculations feel more “human” and Swiftlike.
Make date comparisons more Swiftlike
Let’s start with a new playground and some quick definitions:
 A reference to the user’s calendar,
 an
NSDateFormatter
and format string that makes it easy to define dates in a hurry,  and two dates:
 Valentine’s Day (February 14, 2015 at midnight)
 St. Patrick’s Day (March 17, 2015 at midnight)
Here’s what your code should look like:
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// Playground  noun: a place where people can play import UIKit let userCalendar = NSCalendar.currentCalendar() // Let's create some dates to work with // ==================================== // If you need to instantiate a number of predefined dates, // an NSDateFormatter will get it done in the fewest number of lines. let dateMaker = NSDateFormatter() dateMaker.dateFormat = "yyyy/MM/dd hh:mm:ss Z" // I'm setting these dates according to my time zone (UTC0500); // feel free to adjust the values according to your time zone. let valentinesDay = dateMaker.dateFromString("2015/02/14 00:00:00 05:00")! let stPatricksDay = dateMaker.dateFromString("2015/03/17 00:00:00 05:00")! 
In the previous article, we looked at NSDate's
compare
method, which compares two NSDates
and returns a result of type NSComparisonResult
as shown in the table below:
If…  compare returns… 

the first date is earlier than the second date  .OrderedAscending 
the first date is equal to the second date  .OrderedSame 
the first date is later than the second date  .OrderedDescending 
Add the following code to your playground:
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// (Previous code goes here) // Comparing dates, part 1 // ======================= // First, the clunky NSDate::compare way //  // Returns true because Valentine's Day comes before St. Patrick's Day valentinesDay.compare(stPatricksDay) == .OrderedAscending // Returns true because they're the same date valentinesDay.compare(valentinesDay) == .OrderedSame // Returns true because St. Patrick's Day comes after Valentine's Day stPatricksDay.compare(valentinesDay) == .OrderedDescending 
The compare method works well, but its syntax has that Cstyle clunkiness. It’s a bit jarring in Swift, which has a lot of features that socalled “scripting” languages have. Wouldn’t it be nice if we could compare dates using the ==
, <
, and >
operators?
Let’s make it happen. Add the following code to your playground:
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// (Previous code goes here) // Let's overload the <, >, and == operators to do date comparisons //  func ==(lhs: NSDate, rhs: NSDate) > Bool { return lhs === rhs  lhs.compare(rhs) == .OrderedSame } func <(lhs: NSDate, rhs: NSDate) > Bool { return lhs.compare(rhs) == .OrderedAscending } func >(lhs: NSDate, rhs: NSDate) > Bool { return lhs.compare(rhs) == .OrderedDescending } // Comparisons are less clunky now! //  valentinesDay < stPatricksDay // true valentinesDay == valentinesDay // true stPatricksDay > valentinesDay // true 
With these functions, we’re simply overloading the ==
, <
, and >
operators so that they work on NSDate
s and hide the clunky compare syntax behind some syntactic sugar. In case you’re wondering about the parameter names, lhs
is short for “lefthand side” and rhs
is short for “righthand side”.
Note than in our overload of the ==
operator, there are a couple of ways that two dates can be considered equal:
 Their compare result is
NSComparisonResult.OrderedSame
, or  the two dates being compared are the same
NSDate
object (===
is the identity operator; ifa === b
, thena
andb
both reference the same object).
Make date comparisons more “human”
One recurring theme in science fiction and especially in Star Trek is the tendency for ultrasmart characters and computers to be overly, needlessly, pointlessly precise. The writers for the original series often did this with Spock, and it seemed that at least a few writers were aware of this annoying trope in later series. Here’s a bit of dialogue from The Next Generation:
Data: 6 days, 13 hours, 47 minutes.
Riker: What, no seconds?
Data: I have discovered, sir, a certain level of impatience when I calculate a lengthy time interval to the nearest second. [beat] However if you wish…
Riker: No. No. Minutes is fine.
NSDate
‘s compare method, and, by extension, the ==
, <
, and >
overloads we defined, have the same problem with being overly precise. Let’s consider a case where we have two NSDate
s that are only a second apart:
 Groundhog Day 2015 (February 2, 2015) at 12:00 a.m. EST
 One second after Groundhog Day 2015 at 12:00 a.m. EST
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// (Previous code goes here) // Comparing dates, part 2 // ======================= // Let's create two dates that are only one second apart: let groundhogDay = dateMaker.dateFromString("2015/02/02 00:00:00 05:00")! let groundhogDayPlus1Second = dateMaker.dateFromString("2015/02/02 00:00:01 05:00")! // This line returns false: groundhogDay == groundhogDayPlus1Second // This line returns true: groundhogDay < groundhogDayPlus1Second // NSDate's compare method is too precise for a lot of uses! 
For most purposes, we’d consider midnight on Groundhog Day and one second after midnight Groundhog Day the to be the same time. We need a way to do date comparisons at granularities other than seconds.
If you’re targeting iOS 8 or later, such a way already exists: NSCalendar
‘s compareDate
method! It expects the following parameters:
Parameter  Description 

fromDate 
The first date in the comparison. 
toDate 
The other date in the comparison. 
toUnitGranularity 
The level of precision for the comparison, expressed as an NSCalendarUnit value, which includes:

This is a Cocoa method with the word “compare” in its name, and you’ve probably guessed that its return type is NSComparisonResult
. Here’s what it returns:
If…  compareDate returns… 

fromDate is earlier than toDate , when compared at the specified level of precision 
.OrderedAscending 
fromDate is equal to toDate , when compared at the specified level of precision 
.OrderedSame 
fromDate is later than toDate , when compared at the specified level of precision 
.OrderedDescending 
Let’s try compareDate
out:
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// (Previous code goes here) // Enter NSCalendar's compareDate method, available in iOS 8 and later // This returns false, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // are NOT both within the same SECOND. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Second) == .OrderedSame // This returns true, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // ARE both within the same MINUTE. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Minute) == .OrderedSame // This returns true, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // ARE both within the same HOUR. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Hour) == .OrderedSame // This returns true, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // ARE both within the same DAY. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Day) == .OrderedSame 
Note that compareDate
checks to see if the two given dates are in the same given time period. It doesn’t check to see if the two given dates are separated at most by the given time period. If you’re finding that distinction hard to follow, don’t worry; it’s hard to explain.
It’s easy to demonstrate, however. Suppose we create a new NSDate
that represents one second before Groundhog Day and run some compareDate
tests on it:
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// (Previous code goes here) // 1 second before Groundhog Day, it was the previous day. // Note what happens when we use compareDate: let groundhogDayMinus1Second = dateMaker.dateFromString("2015/02/01 11:59:59 05:00")! // This returns false, because 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // are NOT both within the same SECOND. userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Second) == .OrderedSame // This returns false, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE within a minute of each other, they're not both within the SAME MINUTE // (00:00 vs. 11:59). userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Minute) == .OrderedSame // This returns false, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE within an hour of each other, they're not both within the SAME HOUR // (0 vs. 11). userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Hour) == .OrderedSame // This returns false, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE within a day of each other, they're not both within the SAME DAY // (February 2 vs. February 1). userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Day) == .OrderedSame // This returns true, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE in the same month. userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Month) == .OrderedSame 
Note that compareDate
isn’t available in iOS versions prior to 8. Code targeting iOS 7 or earlier will require writing an equivalent method, which I’ll leave as an exercise for the reader.
Making date arithmetic more Swiftlike
You may have noticed in the code so far that I’ve been creating NSDates
by using an instance of NSDateFormatter
and a defined format string. That’s because this approach uses fewer lines than creating an NSDateComponents
instance, setting its properties, then using a calendar to use the NSDateComponents
instance to create an NSDate
. Unfortunately, there’s no builtin quick way to build an NSDateComponents
instance that represents an interval of time.
This means that answering questions like “What will the date be 1 month, 8 days, 6 hours, and 17 minutes after Groundhog Day?” requires a lot of yak shaving, my favorite term for “tedious settingup”:
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// (Previous code goes here) // Date arithmetic // =============== // The clunky way //  // Suppose we want to get the date of Groundhog Day plus // 1 month, 8 days, 6 hours, and 17 minutes let timeInterval = NSDateComponents() timeInterval.month = 1 timeInterval.day = 8 timeInterval.hour = 6 timeInterval.minute = 17 // The resulting date should be March 10, 2015, 6:17 a.m. let resultDate = userCalendar.dateByAddingComponents(timeInterval, toDate: groundhogDay, options: [])! 
This approach is a clunky ObjectiveCflavored way of doing things. I’d much rather do this calculation with code that looked like this:
1 
let resultDate = groundhogDay + 1.months + 8.days + 6.hours + 17.minutes 
Luckily, we’re working with Swift. Some judicious use of operator overloading and extensions will let us do just that!
First, we need to overload some operators to simplify date component arithmetic:
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// (Previous code goes here) // The neat way //  // First, we define methods that allow us to add and subtract // NSDateComponents instances // The addition and subtraction code is nearly the same, // so we've factored it out into this method func combineComponents(lhs: NSDateComponents, rhs: NSDateComponents, _ multiplier: Int = 1) > NSDateComponents { let result = NSDateComponents() let undefined = Int(NSDateComponentUndefined) result.second = ((lhs.second != undefined ? lhs.second : 0) + (rhs.second != undefined ? rhs.second : 0) * multiplier) result.minute = ((lhs.minute != undefined ? lhs.minute : 0) + (rhs.minute != undefined ? rhs.minute : 0) * multiplier) result.hour = ((lhs.hour != undefined ? lhs.hour : 0) + (rhs.hour != undefined ? rhs.hour : 0) * multiplier) result.day = ((lhs.day != undefined ? lhs.day : 0) + (rhs.day != undefined ? rhs.day : 0) * multiplier) result.month = ((lhs.month != undefined ? lhs.month : 0) + (rhs.month != undefined ? rhs.month : 0) * multiplier) result.year = ((lhs.year != undefined ? lhs.year : 0) + (rhs.year != undefined ? rhs.year : 0) * multiplier) return result } // With combineComponents defined, // overloading + and  is simple func +(lhs: NSDateComponents, rhs: NSDateComponents) > NSDateComponents { return combineComponents(lhs, rhs: rhs) } func (lhs: NSDateComponents, rhs: NSDateComponents) > NSDateComponents { return combineComponents(lhs, rhs: rhs, 1) } // We'll need to overload unary  so we can negate components prefix func (components: NSDateComponents) > NSDateComponents { let result = NSDateComponents() let undefined = Int(NSDateComponentUndefined) if(components.second != undefined) { result.second = components.second } if(components.minute != undefined) { result.minute = components.minute } if(components.hour != undefined) { result.hour = components.hour } if(components.day != undefined) { result.day = components.day } if(components.month != undefined) { result.month = components.month } if(components.year != undefined) { result.year = components.year } return result } 
I derived these functions from Axel Schlueter’s SwiftDateTimeExtensions library. He wrote them when Swift was still in beta; I updated them so that they compile with the current version and added a couple of tweaks of my own. They make it possible to:
 Add the respective
second
,minute
,hour
,day
,month
, andyear
properties of twoNSDateComponent
instances,  subtract the
second
,minute
,hour
,day
,month
, andyear
properties of oneNSDateComponents
instance from the corresponding properties of anotherNSDateComponents
instance, and  negate the
second
,minute
,hour
,day
,month
, andyear
properties of anNSDateComponents
instance.
The addition and subtraction operations are so similar and so tedious; that’s a sign that there’s an opportunity to DRY up the code. That’s why we have the combineComponents
method doing the work and the +
and 
overloads calling it with the right parameters. The combineComponents
code is dense with ternary conditional operators, so I thought I’d explain what’s going on under the hood with this flowchart:
You may be wondering why we defined a negation method and didn’t use it when performing subtraction. That’s because the negation method simply ignores undefined components, while addition and subtraction require treating undefined component values as 0. The negation method comes in handy in other scenarios, which I’ll show later.
Now that we’ve got date component addition, subtraction, and negation defined, let’s extend the Int
type so that it has some instance properties that let us define components with statements like 5.seconds
, 3.minutes
, 7.hours
, 2.days
, 4.weeks
, and so on:
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// (Previous code goes here) // Next, we extend Int to bring some Rubylike magic // to date components extension Int { var seconds: NSDateComponents { let components = NSDateComponents() components.second = self; return components } var second: NSDateComponents { return self.seconds } var minutes: NSDateComponents { let components = NSDateComponents() components.minute = self; return components } var minute: NSDateComponents { return self.minutes } var hours: NSDateComponents { let components = NSDateComponents() components.hour = self; return components } var hour: NSDateComponents { return self.hours } var days: NSDateComponents { let components = NSDateComponents() components.day = self; return components } var day: NSDateComponents { return self.days } var weeks: NSDateComponents { let components = NSDateComponents() components.day = 7 * self; return components } var week: NSDateComponents { return self.weeks } var months: NSDateComponents { let components = NSDateComponents() components.month = self; return components } var month: NSDateComponents { return self.months } var years: NSDateComponents { let components = NSDateComponents() components.year = self; return components } var year: NSDateComponents { return self.years } } 
Once again, I derived these functions from Axel Schlueter’s SwiftDateTimeExtensions library and added a couple of tweaks of my own.
There’s a little redundancy in the code above; it’s to allow for grammatically correct code. I didn’t like seeing code like 1.seconds
, 1.minutes
, 1.hours
, 1.days
, and so on.
With the date component addition and subtraction overloads and the extension to Int
, building date components that represent time intervals is far less tedious:
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// (Previous code goes here) // Building an NSDateComponents instance that represents // a time interval is now a lot nicer: let newTimeInterval = 1.month + 8.days + 6.hours + 17.minutes // Let's confirm that it works newTimeInterval.month // 1 newTimeInterval.day // 8 newTimeInterval.hour // 6 newTimeInterval.minute // 17 
With all our tweaks, adding components to dates using NSCalendar
‘s dateByAddingComponents
feels clunky by comparison. Here are some operator overloads that make this sort of coding more elegant if you’re working with dates and date components expressed in terms of the user’s current calendar:
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// (Previous code goes here) // Let's make it easy to add dates and components, // and subtract components from dates // Date + component func +(lhs: NSDate, rhs: NSDateComponents) > NSDate { return NSCalendar.currentCalendar().dateByAddingComponents(rhs, toDate: lhs, options: [])! } // Component + date func +(lhs: NSDateComponents, rhs: NSDate) > NSDate { return rhs + lhs } // Date  component // (Component  date doesn't make sense) func (lhs: NSDate, rhs: NSDateComponents) > NSDate { return lhs + (rhs) } 
With these methods, date arithmetic now looks like this:
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// (Previous code goes here) // Look at how easy date arithmetic is now: // What's the date and time 2 weeks, 1 day, 13 hours, and 57 minutes // after Groundhog Day 2015? groundhogDay + 2.weeks + 1.day + 13.hours + 57.minutes // (Answer: February 17, 2015, 1:57 p.m.) // Adding dates to date components is quite flexible: 2.weeks + 1.day + 13.hours + 57.minutes + groundhogDay 2.weeks + 1.day + groundhogDay + 13.hours + 57.minutes // What was the date 1 year, 2 months, and 12 days // prior to Groundhog Day 2015? groundhogDay  1.year  2.months  12.days /// (Answer: November 20, 2013) 
That code is so much more pleasant to read (and write!).
And finally, a Ruby on Rails trick comes to Swift
Ruby on Rails lets you do very readable calculations like 2.days.from_now
and 2.days.ago
. We can bring that Rails magic to Swift by using everything we’ve build so far and extending NSDateComponents with two computer properties:
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// (Previous code goes here) // And finally some Ruby on Rails magic, that allows us to create dates // with code like "2.days.fromNow" and "2.days.ago" extension NSDateComponents { var fromNow: NSDate { let currentCalendar = NSCalendar.currentCalendar() return currentCalendar.dateByAddingComponents(self, toDate: NSDate(), options: [])! } var ago: NSDate { let currentCalendar = NSCalendar.currentCalendar() return currentCalendar.dateByAddingComponents(self, toDate: NSDate(), options: [])! } } 
Equipped with everything we’ve made, we can now write code like this:
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// (Previous code goes here) // Let's test the Rails magic! // (It's August 26, 2015, 23:06 as I execute this code) // August 28, 2015, 11:06 p.m. 2.days.fromNow // August 29, 2015, 2:23 a.m. (2.days + 3.hours + 17.minutes).fromNow // August 24, 2015, 11:06 p.m. 2.days.ago // August 24, 2015, 7:49 p.m. (2.days + 3.hours + 17.minutes).ago 
The entire playground
And with that. we’ve got a more Swiftlike way of doing date arithmetic. I’m going to take all these methods and extensions and post them as a library on GitHub, but in the meantime, here’s the complete playground for the exercises in this article. Go forth and write some readable date/time code!
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// Date/time overloads and extensions Swift playground // by Joey deVilla  August 2015 // Partially based on Axel Schlueter's SwiftDateTimeExtensions library // (https://github.com/schluete/SwiftDateTimeExtensions) // // Released under the MIT License (MIT) // (c) 2015 Joey deVilla import UIKit let userCalendar = NSCalendar.currentCalendar() // Let's create some dates to work with // ==================================== // If you need to instantiate a number of predefined dates, // an NSDateFormatter will get it done in the fewest number of lines. let dateMaker = NSDateFormatter() dateMaker.dateFormat = "yyyy/MM/dd hh:mm:ss Z" // I'm setting these dates according to my time zone (UTC0500); // feel free to adjust the values according to your time zone. let valentinesDay = dateMaker.dateFromString("2015/02/14 00:00:00 05:00")! let stPatricksDay = dateMaker.dateFromString("2015/03/17 00:00:00 05:00")! // Comparing dates, part 1 // ======================= // First, the clunky NSDate::compare way //  // Returns true because Valentine's Day comes before St. Patrick's Day valentinesDay.compare(stPatricksDay) == .OrderedAscending // Returns true because they're the same date valentinesDay.compare(valentinesDay) == .OrderedSame // Returns true because St. Patrick's Day comes after Valentine's Day stPatricksDay.compare(valentinesDay) == .OrderedDescending // Let's overload the <, >, and == operators to do date comparisons //  func ==(lhs: NSDate, rhs: NSDate) > Bool { return lhs === rhs  lhs.compare(rhs) == .OrderedSame } func <(lhs: NSDate, rhs: NSDate) > Bool { return lhs.compare(rhs) == .OrderedAscending } func >(lhs: NSDate, rhs: NSDate) > Bool { return lhs.compare(rhs) == .OrderedDescending } // Comparisons are less clunky now! //  valentinesDay < stPatricksDay // true valentinesDay == valentinesDay // true stPatricksDay > valentinesDay // true // Comparing dates, part 2 // ======================= // Let's create two dates that are only one second apart: let groundhogDay = dateMaker.dateFromString("2015/02/02 00:00:00 05:00")! let groundhogDayPlus1Second = dateMaker.dateFromString("2015/02/02 00:00:01 05:00")! // This line returns false: groundhogDay == groundhogDayPlus1Second // This line returns true: groundhogDay < groundhogDayPlus1Second // NSDate's compare method is too precise for a lot of uses! // Enter NSCalendar's compareDate method, available in iOS 8 and later // This returns false, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // are NOT both within the same SECOND. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Second) == .OrderedSame // This returns true, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // ARE both within the same MINUTE. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Minute) == .OrderedSame // This returns true, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // ARE both within the same HOUR. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Hour) == .OrderedSame // This returns true, because 2015/02/02 00:00:00 and 2015/02/02 00:00:01 // ARE both within the same DAY. userCalendar.compareDate(groundhogDay, toDate: groundhogDayPlus1Second, toUnitGranularity: .Day) == .OrderedSame // 1 second before Groundhog Day, it was the previous day. // Note what happens when we use compareDate: let groundhogDayMinus1Second = dateMaker.dateFromString("2015/02/01 11:59:59 05:00")! // This returns false, because 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // are NOT both within the same SECOND. userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Second) == .OrderedSame // This returns false, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE within a minute of each other, they're not both within the SAME MINUTE // (00:00 vs. 11:59). userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Minute) == .OrderedSame // This returns false, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE within an hour of each other, they're not both within the SAME HOUR // (0 vs. 11). userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Hour) == .OrderedSame // This returns false, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE within a day of each other, they're not both within the SAME DAY // (February 2 vs. February 1). userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Day) == .OrderedSame // This returns true, because while 2015/02/02 00:00:00 and 2015/02/01 11:59:59 // ARE in the same month. userCalendar.compareDate(groundhogDay, toDate: groundhogDayMinus1Second, toUnitGranularity: .Month) == .OrderedSame // Date arithmetic // =============== // The clunky way //  // Suppose we want to get the date of Groundhog Day plus // 1 month, 8 days, 6 hours, and 17 minutes let timeInterval = NSDateComponents() timeInterval.month = 1 timeInterval.day = 8 timeInterval.hour = 6 timeInterval.minute = 17 // The resulting date should be March 10, 2015, 6:17 a.m. let resultDate = userCalendar.dateByAddingComponents(timeInterval, toDate: groundhogDay, options: [])! // The neat way //  // First, we define methods that allow us to add and subtract // NSDateComponents instances // The addition and subtraction code is nearly the same, // so we've factored it out into this method func combineComponents(lhs: NSDateComponents, rhs: NSDateComponents, _ multiplier: Int = 1) > NSDateComponents { let result = NSDateComponents() let undefined = Int(NSDateComponentUndefined) result.second = ((lhs.second != undefined ? lhs.second : 0) + (rhs.second != undefined ? rhs.second : 0) * multiplier) result.minute = ((lhs.minute != undefined ? lhs.minute : 0) + (rhs.minute != undefined ? rhs.minute : 0) * multiplier) result.hour = ((lhs.hour != undefined ? lhs.hour : 0) + (rhs.hour != undefined ? rhs.hour : 0) * multiplier) result.day = ((lhs.day != undefined ? lhs.day : 0) + (rhs.day != undefined ? rhs.day : 0) * multiplier) result.month = ((lhs.month != undefined ? lhs.month : 0) + (rhs.month != undefined ? rhs.month : 0) * multiplier) result.year = ((lhs.year != undefined ? lhs.year : 0) + (rhs.year != undefined ? rhs.year : 0) * multiplier) return result } // With combineComponents defined, // overloading + and  is simple func +(lhs: NSDateComponents, rhs: NSDateComponents) > NSDateComponents { return combineComponents(lhs, rhs: rhs) } func (lhs: NSDateComponents, rhs: NSDateComponents) > NSDateComponents { return combineComponents(lhs, rhs: rhs, 1) } // We'll need to overload unary  so we can negate components prefix func (components: NSDateComponents) > NSDateComponents { let result = NSDateComponents() let undefined = Int(NSDateComponentUndefined) if(components.second != undefined) { result.second = components.second } if(components.minute != undefined) { result.minute = components.minute } if(components.hour != undefined) { result.hour = components.hour } if(components.day != undefined) { result.day = components.day } if(components.month != undefined) { result.month = components.month } if(components.year != undefined) { result.year = components.year } return result } // Next, we extend Int to bring some Rubylike magic // to date components extension Int { var seconds: NSDateComponents { let components = NSDateComponents() components.second = self; return components } var second: NSDateComponents { return self.seconds } var minutes: NSDateComponents { let components = NSDateComponents() components.minute = self; return components } var minute: NSDateComponents { return self.minutes } var hours: NSDateComponents { let components = NSDateComponents() components.hour = self; return components } var hour: NSDateComponents { return self.hours } var days: NSDateComponents { let components = NSDateComponents() components.day = self; return components } var day: NSDateComponents { return self.days } var weeks: NSDateComponents { let components = NSDateComponents() components.day = 7 * self; return components } var week: NSDateComponents { return self.weeks } var months: NSDateComponents { let components = NSDateComponents() components.month = self; return components } var month: NSDateComponents { return self.months } var years: NSDateComponents { let components = NSDateComponents() components.year = self; return components } var year: NSDateComponents { return self.years } } // Building an NSDateComponents instance that represents // a time interval is now a lot nicer: let newTimeInterval = 1.month + 8.days + 6.hours + 17.minutes // Let's confirm that it works newTimeInterval.month // 1 newTimeInterval.day // 8 newTimeInterval.hour // 6 newTimeInterval.minute // 17 // Let's make it easy to add dates and components, // and subtract components from dates // Date + component func +(lhs: NSDate, rhs: NSDateComponents) > NSDate { return NSCalendar.currentCalendar().dateByAddingComponents(rhs, toDate: lhs, options: [])! } // Component + date func +(lhs: NSDateComponents, rhs: NSDate) > NSDate { return rhs + lhs } // Date  component // (Component  date doesn't make sense) func (lhs: NSDate, rhs: NSDateComponents) > NSDate { return lhs + (rhs) } // Look at how easy date arithmetic is now: // What's the date and time 2 weeks, 1 day, 13 hours, and 57 minutes // after Groundhog Day 2015? groundhogDay + 2.weeks + 1.day + 13.hours + 57.minutes // (Answer: February 17, 2015, 1:57 p.m.) // Adding dates to date components is quite flexible: 2.weeks + 1.day + 13.hours + 57.minutes + groundhogDay 2.weeks + 1.day + groundhogDay + 13.hours + 57.minutes // What was the date 1 year, 2 months, and 12 days // prior to Groundhog Day 2015? groundhogDay  1.year  2.months  12.days /// (Answer: November 20, 2013) // And finally some Ruby on Rails magic, that allows us to create dates // with code like "2.days.fromNow" and "2.days.ago" extension NSDateComponents { var fromNow: NSDate { let currentCalendar = NSCalendar.currentCalendar() return currentCalendar.dateByAddingComponents(self, toDate: NSDate(), options: [])! } var ago: NSDate { let currentCalendar = NSCalendar.currentCalendar() return currentCalendar.dateByAddingComponents(self, toDate: NSDate(), options: [])! } } // Let's test the Rails magic! // (It's August 26, 2015, 23:06 as I execute this code) // August 28, 2015, 11:06 p.m. 2.days.fromNow // August 29, 2015, 2:23 a.m. (2.days + 3.hours + 17.minutes).fromNow // August 24, 2015, 11:06 p.m. 2.days.ago // August 24, 2015, 7:49 p.m. (2.days + 3.hours + 17.minutes).ago 
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{ 2 comments… read them below or add one }
Great Post. Very detailed but I am a newbie and i’m working on an app in swift that requires that i get the difference between three dates using datepicker and labels and divide it by an integer. I was wondering if you could point me in the right direction.
Here are the details:
I have 3 UIDatePickers when user scrolls picker and selects a date. The date populates in a textLabel. Then i need to do some calculations with the dates picked by the user which populates in the textLabels. I am solving for Q5
The logic is if (f5 <q2 ) then (c5q2r2) / 7 else (c5f5r2) / 7
let dateMaker = NSDateFormatter()
dateMaker.dateFormat = "MM/dd/yyyy"
var f5 = dateMaker.dateFromString("\(futureDateLabel)")
var q2 = dateMaker.dateFromString("\(StartSchoolDateLabel)")
var c5 = dateMaker.dateFromString("\(StrClassDateLabel)")
var divisor = 7
var days2Complete = daysToComplete.text.toInt()
var Q5 = 0
let userCalendar = NSCalendar.currentCalendar()
if (f5?.earlierDate(q2!) != nil)
{
let dayCalendarUnit:NSCalendarUnit = .DayCalendarUnit
var Difference = userCalendar.components(dayCalendarUnit, fromDate: c5!, toDate: q2!, options: nil)
Q5 = (Difference.day – days2Complete!) / divisor
}
else{
let dayCalendarUnit:NSCalendarUnit = .DayCalendarUnit
var Difference = userCalendar.components(dayCalendarUnit, fromDate: c5!, toDate: f5!, options: nil)
Q5 = (Difference.day – days2Complete!) / divisor
}
when i run it the app crashes on the else statement but when i comment out the else statement the app runs but Q5 populates as 0 in the textbox. I have been trying to solve this for days and i have read all three post of yours in regards to dates and arithmetic and I'm still stuck. Please help a newbie out at your convenience. Hope to hear from you soon. Thanks
Wonderful series of articles! Exactly sync’ed with what I was just beginning to research. Thank you very much for your time and effort.
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