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swiftdate-formatting

Swift date manipulation - strange month return

I'm trying to do some date manipulation with Swift and I'm getting an unexpected result. The webservice will pass in a string date, and then I want to get that month and the previous month. I'm using this code (with input grab and such removed):

import Foundation

let formatter = DateFormatter()
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(identifier: "America/New_York")
formatter.dateFormat = "yyyy-MM-dd"

let date = formatter.date(from: "2018-12-01")!
let prev = Calendar.current.date(byAdding: .month, value: -1, to: date)!

formatter.string(from: date)
formatter.string(from: prev)

So I've got a valid date, and then I subtract a month from it. The first formatted date shows my expected 2018-12-01 but then on the second line, instead of saying 2018-11-01 it says 2018-10-31.

I'm in PST, which is of course 3 hours ahead of EST. If I add 3 hours I'd get the expected strings. However, since both the input and the output strings were done with a formatter using the timezone, why don't I get the expected output?

Solution

  • The problem is that Calendar.current is in a different timezone (for you) than the formatter.

    So date is December 1, 2018 at midnight New York time. But that is November 30, 2018 at 9pm local time (PST) for you.

    When you subtract one month it is done in local time (Calendar.current) so you get October 30, 2018 at 9pm. Then you format that date to New York time and it results in October 31, 2018 at midnight.

    To get the proper results you want a Calendar in the same timezone as the formatter:

    var cal = Calendar(identifier: Calendar.current.identifier)
cal.timeZone = formatter.timeZone
let prev = cal.date(byAdding: .month, value: -1, to: date)!

    This will give the expected result.