How to work with dates and times in Swift 3, part 1: Dates, Calendars, and DateComponents

by Joey deVilla on August 18, 2016

i just want to use dates

If you’re just getting started with date and time programming in Swift 3, chances are that you probably did some Googling, found something about NSDate and its companion classes in Apple’s documentation and promptly got confused. Let me reassure you that it isn’t your fault.

For starters, a lot of the documentation out there is in Objective-C, which can throw you off if you’re not familiar with its [instance method: parameter2: parameter3:] method-calling syntax. There’s also the fact that Apple recently removed the NS prefix from Cocoa’s class names — NSDate is now just plain ol’ Date, NSCalendar is now Calendar, NSDateComponents is now DateComponents, and so on. And finally, in the move towards protocol-oriented programming, Apple has changed some classes to structs, including many of the classes for working with dates and times.

And finally, if you’re coming to Swift 3 from JavaScript, which makes do with a single object type called Date, the idea of having this set of classes just to handle dates and times looks like overkill:

swift 3 date and time classes

Click the diagram to see it at full size.

In this series of articles, we’ll look at date and time programming in Swift 3. By its end, you’ll have a firm grasp on the topic.

Swift’s Date struct represents dates and times

swift date struct

Click the diagram to see it at full size.

In Swift, dates and times are represented by instances of Date, which is a struct. Date is independent of any time zone, or even any calendar system. It gets this independence through the way it represents time: as a number of seconds relative to the start of the Third Millennium, January 1, 2001, 00:00:00 UTC.

The following playground code shows the various ways of creating Dates:

Of course, we don’t think of dates and times in terms of seconds relative to the start of the Third Millennium, or the start of the Unix Epoch, or any other arbitrary date and time. That’s why Swift features a couple of other structs to help us make sense of Dates: Calendar and DateComponents.

Calendars give dates context, DateComponents let us assemble dates or break dates apart

swift calendar struct

Click the diagram to see it at full size.

Think of the Calendar struct as a way to view Dates in a way that makes more sense to us: not as a number of seconds before or after January 1, 2001 00:00:00 UTC, but in terms of a year, month, day, hour, minute, and more.

The Calendar struct supports 16 different calendar systems, including the Gregorian calendar (a.k.a. the Western or Christian calendar), which is likely the one you use the most. Consider Swift’s reference date:

  • In a Date struct, its value is 0.
  • In the Gregorian calendar, this date has the following values:
    • Year: 2001
    • Month: 1
    • Day: 1
    • Hour: 0
    • Minute: 0
  • In the Hebrew calendar, this date has the following values:
    • Year: 5761
    • Month: 4
    • Day: 6
    • Hour: 0
    • Minute: 0
  • In the Buddhist calendar, this date has the following values:
    • Year: 2543
    • Month: 1
    • Day: 1
    • Hour: 0
    • Minute: 0

In most apps, you’ll likely be using the Gregorian calendar, so Calendar‘s real use is to convert Dates into DateComponents, and DateComponents into Dates:

swift datecomponents struct

Click the diagram to see it at full size.

The DateComponents struct is an assembly of properties that make up a date, such as year, month, date, hour, minute, second, and so on. DateComponents instances can be used to represent either a specific point in time, or a duration of time.

Using both Calendar and DateComponents structs, we can perform these key actions:

  • Build Dates using properties such as year, month, day, hour, and minute rather than a number of second relative to a reference date, and
  • extract properties from Dates, such as year, month, day, hour, and minute.

Let’s start creating some Dates with the help of Calendar and DateComponents.

Let’s create a Date given a year, month, and day, part 1

alexander graham bell

Let’s start with the first date in phone history: March 10, 1876, the day when Alexander Graham Bell made the first phone call. Create a new playground and enter or paste the code below:

In the code, we:

  • Get the user’s current Calendar.
  • Create an DateComponents struct, firstLandPhoneCallDateComponents, providing values for the year, month, and day parameters, and nil for all the others.
  • Use the user’s Calendar to create firstLandPhoneCallDate using firstLandPhoneCallDateComponents.
  • Get the internal representation of the Date.

Here’s a screenshot of the playground code as seen on my computer, whose time zone is set to “US/Eastern” (UTC-5):

firstlandphonecalldate

Click the screenshot to see it at full size.

Note the results in the sidebar at the lower right-hand corner:

  • firstLandPhoneCallDate‘s value corresponds to the DateComponents properties we set: March 10, 1876, 12:00 a.m..
  • firstLandPhoneCallDate‘s internal value, contained within its timeIntervalSinceReferenceDate property, is -3,938,697,748, which indicates that Alexander Graham Bell’s inaugural phone call was made nearly 4 billion seconds prior to the start of the Third Millennium.

Let’s create a Date given a year, month, and day, part 2

martin cooper

Let’s try creating another momentous date in phone history: the day when Martin Cooper made the first cellular phone call, April 3, 1973. We’ll do it differently this time, by creating a blank DateComponents struct, and then setting its year, month, and day properties. Add the following code to the code above:

In the code, we:

  • Create an empty DateComponents struct, firstCellPhoneCallDateComponents.
  • Set the year, month, and day properties of firstCellPhoneCallDateComponents to correspond to the date April 3, 1973.
  • Use the user’s Calendar to create firstCellPhoneCallDate using firstCellPhoneCallDateComponents.
  • Get the internal representation of the Date.

Here’s a screenshot of the results:

firstcellphonecalldate

Click the screenshot to see it at full size.

Let’s create a Date: What date does National Donut Day — the first Friday in June — fall on in 2017?

Having come from Canada, the country with the world’s highest per capita donut shop concentration and the people who eat the most donuts per capita, I can assure you that National Donut Day has been a real thing since 1938. It takes place on the first Friday in June, and we can find out what date it falls on in 2017 — or any other year — through the judicious use of DateComponents properties.

Add the following code to the current playground:

You should be familiar with the year and month DayComponents properties by now, and we’re using a couple that may be new to you:

  • weekday: Specifies a day of the week. With the Gregorian calendar, valid values are 1 through 7, where 1 is Sunday, 2 is Monday, 3 is Tuesday, and so on. Since we’re looking for a Friday, we’ve set this value to 6.
  • weekdayOrdinal: Specifies the order of the given weekday in the next larger specified calendar unit. Since we set weekday to 6 and set this value to 1, and since the next largest specified calendar unit was month, we’ll get the date of the first Friday of the month.

If you check the value of donutDayDate in the playground’s sidebar, you should see Jun 2, 2017, 12:00 AM. If you look at a calendar, you’ll confirm that it is indeed the first Friday of June 2017.

changing stuff and seing what happens

In the spirit of the fake book cover shown above, let’s see what happens if we don’t specify the month. Comment out the following line from the code you just added…

donutDayComponents.month = 6

…so that the code now looks like this:

When you run the code, you’ll see that the date for donutDayDate is now Jan 6 2017, 12:00 AM. Now that we’re specifying only a year and not a month, Swift interprets the combination of donutDayComponents.weekday = 6 and donutDayComponents.weekdayOrdinal = 1 to mean “the first Friday of the year”. If you look at a calendar, you’ll confirm that January 6, 2017 is indeed the first Friday of the year.

Let’s create one more Date: 5:00.pm. on Thursday of the 18th week of 2017…in Tokyo.

The great thing about Swift’s Calendar class is that it does its best to work with the DateComponents that you give it, and DateComponents gives you all sorts of ways to specify a date. Let’s assume for a moment that you’re in Japan and want to leave work at the ridiculously early hour of 5 p.m. and have some relaxing Suntory times, and it just happens to be the 18th week of 2017. What’s the date?

The answer comes from this code:

On my system, which is in the Eastern daylight time zone (UTC-4) at the time of writing, thursday5pm18thWeek2017TokyoDate displays as May 4, 2017, 4:00 AM in my playground’s sidebar, and looking at a calendar confirms that May 4th is indeed the Thursday of the 18th month of 2017.

Let’s extract DateComponents from a Date, part 1

Now that we’ve created some Dates using DateComponents, let’s do the reverse and extract DateComponents from given Dates. We’ll continue with our playground and use a Date we’ve already created: firstLandPhoneCallDate, which corresponds to the date of Alexander Graham Bell’s historic phone call, March 10, 1876. Here’s code that extracts the year, month, and day from this Date:

Let’s extract DateComponents from a Date, part 2

This time, let’s create a new Date — one that corresponds to this key date in iOS history: the “Stevenote” where the original iPhone was first announced:

If you were to ask Swift when this Stevenote took place, it would reply “190,058,400 seconds after the reference date”. For most of us, this is a meaningless figure, so we’ll extract the following DateComponents from this Date:

  • Year
  • Month
  • Day
  • Hour
  • Minute
  • What day of the week this Date fell on
  • What week of the year this Date fell on

Here’s the code:

Let’s extract DateComponents from a Date, part 3

Let’s try it again with another key iOS date — the Stevenote where the original iPad was announced:

This time, if you were to ask Swift when this Stevenote took place, it would reply “286,308,000 seconds after the reference date”. Let’s get all the DateComponents for this date:

Let’s take a look at each DateComponents property and what it represents:

Property Description
calendar The calendar system for the date represented by this set of DateComponents. We got these DateComponents by converting a Date using a Gregorian Calendar, so in this case, this value is gregorian.
day The day number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 27.
era The era for this particular date, which depends on the date’s calendar system. In this case, we’re using the Gregorian calendar, which has two eras:

  • BCE (a.k.a. BC), represented by the integer value 0
  • CE (a.k.a. AD), represented by the integer value 1
hour The hour number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 13, because in my time zone, 18:00:00 UTC is 13:00:00.
minute The minute number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 0.
month The month number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 1.
nanosecond The nanosecond number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 0.
quarter The quarter number of this particular date and time. January 27, 2010, 18:00:00 UTC, is in the first quarter of the year, so this value is 0.
second The second number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 0.
timeZone The time zone of this particular date and time. I’m in the UTC-5 time zone (US Eastern), so this value is set to that time zone.
weekday The day of the week of this particular date and time. In the Gregorian calendar, Sunday is 1, Monday is 2, Tuesday is 3, and so on. January 27, 2010, was a Wednesday, so this value is 4.
weekdayOrdinal The position of the weekday within the next larger specified calendar unit, which in this case is a month. So this specifies nth weekday of the given month. Jauary 27, 2010 was on the 4th Wednesday of the month, so this value is 4.
weekOfMonth The week of the month of this particular date and time. January 27, 2010 fell on the 5th week of January 2010, so this value is 5.
weekOfYear The week of the year of this particular date and time. January 27, 2010 fell on the 5th week of 2010, so this value is 5.
year The year number of this particular date and time. For January 27, 2010, 18:00:00 UTC, this value is 2010.
yearForWeekOfYear Oh wow, this is so hard to explain that I’ll leave it to Apple’s docs.

Wrapping it all up

Here’s the playground containing all the code we just worked with:

In the next installment, we’ll look at converting Dates to Strings, and vice versa.

{ 5 comments… read them below or add one }

1 alex September 15, 2016 at 9:47 am

in c# very simple

DateTime now = DateTime.Now;

Apple is really son of a bitch…

I am trying to print date time for one hour in swift 3….

Is it not possible to do that?

2 Joey deVilla September 15, 2016 at 1:39 pm

Here’s how:

let now = Date()
var myDateFormatter = DateFormatter()
myDateFormatter.dateStyle = .medium
print(myDateFormatter.string(from: now))
3 Gilles October 26, 2016 at 1:45 am

Amazing.

4 Dave June 18, 2017 at 7:06 am

Great tutorial thanks.

5 Lu Hao July 11, 2017 at 10:16 am

Best tutorial on dates I have seen so far! Thank you!!

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