How can I ignore time zones in Swift Dates? - swift

Question

I wish to use Swift 5 to create a Date object with the day, month, and year I pass it. Problem is that the DateFormatter has its own ideas, and seems to be treating my Date objects as if they were UTC even whatever I set <formatter>.timeZone = ... to.
Say I want t date object with the date the first of April.
private func firstOfApril() -> String {
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
return "\(formatter.date(from: "2021-04-01")!)"
}
If I do this in the AM here (GMT + 13) I get 2021-03-31 11:00:00 +0000. I expect: 2021-04-01 00:00:00
WHat I have tried:
enum TZType{
case None
case Current
case Auto
case Nil
case Default
}
private func firstOfApril(none_current:TZType) -> String {
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
switch none_current {
case .None:
formatter.timeZone = .none
case .Current:
formatter.timeZone = .current
case .Nil:
formatter.timeZone = nil
case .Auto:
formatter.timeZone = .autoupdatingCurrent
case .Default:
break
}
return "\(formatter.date(from: "2021-04-01")!)"
}
print(firstOfApril(none_current: .None))
print(firstOfApril(none_current: .Current))
print(firstOfApril(none_current: .Auto))
print(firstOfApril(none_current: .Nil))
print(firstOfApril(none_current: .Default))
Outputs
2021-03-31 11:00:00 +0000
2021-03-31 11:00:00 +0000
2021-03-31 11:00:00 +0000
2021-03-31 11:00:00 +0000
There is no concept of time zones in my application. I think there should be, but I am overruled. All time is "local time", as in wall clock time, not local time zone.

Answer 1

Leo got it.
The last lines of that function should be:
let retDate = formatter.date(from: "2021-04-01")!
return formatter.string(from: retDate)
In total:
private func firstOfApril(none_current:TZType) -> String {
let formatter = DateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
switch none_current {
case .None:
formatter.timeZone = .none
case .Current:
formatter.timeZone = .current
case .Nil:
formatter.timeZone = nil
case .Auto:
formatter.timeZone = .autoupdatingCurrent
case .Default:
break
}
let retDate = formatter.date(from: "2021-04-01")!
return formatter.string(from: retDate)
}

Answer 2

You've gotten some good, but incomplete, information.
A Date object captures a moment in time, anywhere on the planet. It is devoid of a time-zone. (Internally, it's recorded by a number of seconds since the iOS/MacOS "epoch date", or "zero date". That zero date is in UTC.)
Date objects don't have a time zone, but they also don't have a fixed time or date. The same instant in time will be at different times of day in different time zones, and may even be different dates (e.g. It is about 7 PM on Monday 29 March here in the US Eastern Daylight Time timezone. It is about midnight on Tuesday 30 March in London.)
If you create a date formatter and feed it a month/day/year, it will default to creating a Date object using your local time zone. It's not the Date object that has a time zone, it's the date formatter.
You can either use the date formatter you already created to convert you dates back to strings (using the DateFormatter function string(from:), or you can use a DateFormatter class function called localizedString(from:dateStyle:timeStyle:. That function takes a date and displays it in your local time zone, using the conventions of your locale, and lets you specify short, medium, or long format dates.
If you write
let date = Date()
print(DateFormatter.localizedString(from: date, dateStyle: .medium, timeStyle: .medium))
You'll get something like this:
Mar 29, 2021 at 7:07:13 PM
That is expressed in my local time zone, using US date and time formats.
(But with the date and time at which you run it, and using the date and time formatting conventions for your locale.)
Next, it's how you display the date. If you write the code
print(Date())
What you will see is the date at the instant you run the code, but expressed in UTC. For me, that will be 4 hours ahead of my current time, so it will claim it's about 23:00 on 30 January.
2021-03-29 23:00:48 +0000
If you are in China, and I am in the US, and our phones' clocks are both synchronized exactly, and we capture the current date using Date() at the same instant, we will get the exact same Date value. If we were to both express it in UTC, it would show the same date and time. (The Date object doesn't have a time