1st april dates of 80s failed to parse in iOS 10.0

Question

I found that DateFormatter date(from:) method can't parse a couple of specific dates. Method returns nil for the 1st april of 1981-1984 years. Is it a bug of Foundation? What can we do to perform parsing of such dates?

Xcode 8.0, iOS SDK 10.0. Here is a screenshot of a short playground example:

Answer 1

This problem occurs if daylight saving time starts exactly on midnight, as it was the case in Moscow in the years 1981–1984 (see for example Clock Changes in Moscow, Russia (Moskva)).

This was also observed in

• Why does NSDateFormatter return nil date for these 4 time zones? and
• Why NSDateFormatter is returning null for a 19/10/2014 in a Brazilian time zone?

For example, at midnight of April 1st 1984, the clocks were adjusted one hour forward, which means that the date "1984-04-01 00:00" does not exist in that timezone:

let dFmt = DateFormatter()
dFmt.dateFormat = "yyyy-MM-dd"
dFmt.timeZone = TimeZone(identifier: "Europe/Moscow")
print(dFmt.date(from: "1984-04-01")) // nil

As a solution, you can tell the date formatter to be "lenient":

dFmt.isLenient = true

and then it will return the first valid date on that day:

dFmt.isLenient = true
if let date = dFmt.date(from: "1984-04-01") {
    dFmt.dateFormat = "yyyy-MM-dd HH:mm:ss"
    print(dFmt.string(from: date)) 
}
// 1984-04-01 01:00:00

A different solution was given by rob mayoff, which is to make the date formatter use noon instead of midnight as the default date. Here is a translation of rob's code from Objective-C to Swift:

let noon = DateComponents(calendar: dFmt.calendar, timeZone: dFmt.timeZone,
               year: 2001, month: 1, day: 1, hour: 12, minute: 0, second: 0)
dFmt.defaultDate = noon.date
if let date = dFmt.date(from: "1984-04-01") {
    dFmt.dateFormat = "yyyy-MM-dd HH:mm:ss"
    print(dFmt.string(from: date)) 
}
// 1984-04-01 12:00:00