Parsing Dates Without Times

Parsing fixed-format date strings is tricky. For an explanation as to why, see QA1480 NSDateFormatter and Internet Dates. However, there’s an extra wrinkle when you try to parse fixed-format date strings that don’t include a time. Consider this code:

Code Block  
func startOfDateInSaoPaulo(_ dateStr: String) -> Date? {
		let df = DateFormatter()
		df.locale = Locale(identifier: "en_US_POSIX")
		df.timeZone = TimeZone(identifier: "America/Sao_Paulo")!
		df.dateFormat = "yyyyMMdd"
		return df.date(from: dateStr)
}

The goal here is to parse a string of the form yyyyMMdd and return the start of that day in São Paulo, Brazil. And the code seems to work. For example:

Code Block  
print(startOfDateInSaoPaulo("20200722")?.description ?? "nil")
// -> 2020-07-22 03:00:00 +0000

It even handles daylight saving time changes. São Paulo set the clocks forward on 4 Nov 2018, and you can see this change when you map 3 Nov and 5 Nov:

Code Block  
print(startOfDateInSaoPaulo("20181103")?.description ?? "nil")
// -> 2018-11-03 03:00:00 +0000
print(startOfDateInSaoPaulo("20181105")?.description ?? "nil")
// -> 2018-11-05 02:00:00 +0000

Note Time zones in Brazil are very exciting. If you’re curious, read Time in Brazil and Daylight saving time in Brazil. Also, for those Northern hemisphere folks out there, keep in mind that São Paulo is in the southern hemisphere and thus the daylight saving “spring forward” happens in the second half of the year.

However, consider this:

Code Block  
print(startOfDateInSaoPaulo("20181104")?.description ?? "nil")
// -> nil

Whoah!?! This is failing because, internally, the date formatter maps the date string to a set of date components (DateComponents). The year, month, and day come from the date string, but the hour, minute, and second default to 0. In Brazil, daylight saving time starts at midnight, and thus the date components year: 2018, month: 11, day: 4, hour: 0, minute: 0, second: 0 don’t exist in the São Paulo time zone. When the date formatter runs these components through the calendar, it returns nil.

Note Making daylight saving time changes at midnight is weird but Brazil is not the only country that does this.

The solution here is to set the defaultDate property on the date formatter to something in the middle of the day. For example:

Code Block  
func middleOfDateInSaoPaulo(_ dateStr: String) -> Date? {
		let df = DateFormatter()
		// This translates to midday, Sao Paulo time, on 2001-01-01.
		df.defaultDate = Date(timeIntervalSinceReferenceDate: 50400.0)
		df.locale = Locale(identifier: "en_US_POSIX")
		df.timeZone = TimeZone(identifier: "America/Sao_Paulo")!
		df.dateFormat = "yyyyMMdd"
		return df.date(from: dateStr)
}

Now all the dates centred around 4 Nov 2018 work just fine:

Code Block  
print(middleOfDateInSaoPaulo("20181103")?.description ?? "nil")
// -> 2018-11-03 15:00:00 +0000
print(middleOfDateInSaoPaulo("20181104")?.description ?? "nil")
// -> 2018-11-04 14:00:00 +0000
print(middleOfDateInSaoPaulo("20181105")?.description ?? "nil")
// -> 2018-11-05 14:00:00 +0000

If necessary, you can call startOfDay(for:) to get the a Date value that represents the start of the day.

In many cases, however, it’s better to avoid this problem by working with a set of date components rather than a date. Remember that a Date value represents an absolute point in time, and that’s not always the best model for a day. For example, if you’re storing someone’s birthday, it’s better to store date components rather than a date.

And this brings us back to a key piece of advice from QA1480:

look at solutions outside of the Cocoa space

If your final goal is to get a set of date components, you don’t need a date formatter at all. Rather, simply parse the fixed-format string yourself:

Code Block  
func dateComponentsOfDate(_ dateStr: String) -> DateComponents? {
		let yearStr = dateStr.prefix(4)
		let monthStr = dateStr.dropFirst(4).prefix(2)
		let dayStr = dateStr.dropFirst(6).prefix(2)
		guard
				let year = Int(yearStr),
				let month = Int(monthStr),
				let day = Int(dayStr)
		else { return nil }
		return DateComponents(year: year, month: month, day: day)
}
print(dateComponentsOfDateInSaoPaulo("20181104")?.description ?? "nil")
// -> year: 2018 month: 11 day: 4 …

So, to summarise:

• Converting fixed-format strings that represent a date without a time is tricky.

• Unless you can guarantee that you’re working in a time zone that has not, and will never, do daylight saving time changes at midnight, you must use the defaultDate technique shown above.

• Alternatively, you can avoid this problem entirely by working with date components rather than dates.

I found this explanation very useful in the context of debugging an off-by-one-day bug in my crossword app. It's a classic scenario where crosswords are referenced by date but without a specific time, except that the date is usually reckoned by the time zone of the host publication.

I thought I would share an example of how I'm addressing the issue using the defaultDate technique, but generalizing it to choose a "middle of the day" time that should work with whatever time zone (Quinn's example uses a fixed relative time interval that is specific to Sao Paolo). Given a date formatter that already has the desired time zone set on it:

var midDayComponents = DateComponents(hour: 12)
midDayComponents.calendar = Calendar(identifier: .gregorian)
midDayComponents.timeZone = dateFormatter.timeZone
dateFormatter.defaultDate = midDayComponents.date

It seems that all you need is an hour, a calendar, and a time zone to come up with a suitably mid-day date. I hope this helps somebody!

Thanks for sharing Daniel. Also, for reminding me that I meant to lock this thread when I created it. Fixing that now (-:

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"