Swift 5 Datetime Conversion triple timezone cast (???) - date

Question

I have a DateTime that I'm reading from an API that is in GMT.
I want to force cast it to an EST date object.
The original object is a string that I then cast to a Date to do some time comparison. Unfortunately, I don't think I did this right which made me do this unholy abomination:
extension String {
//this force casts from our original data being in GMT to EST
func getDateTimeTZ() -> Date{
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd HH:mm"
dateFormatter.timeZone = TimeZone(abbreviation: "GMT")
if let date = dateFormatter.date(from: self) {
dateFormatter.timeZone = TimeZone(abbreviation: "EST")
let localTime = dateFormatter.string(from: date)
dateFormatter.timeZone = TimeZone(abbreviation: "GMT")
return dateFormatter.date(from: localTime)!
}
return dateFormatter.date(from: self)!
}
}
For whatever reason, I seem to have to flip the dateformatter 3 times to get the correct EST string output. Is there a better way to do this?

Answer 1

A Date is not associated with a time zone. From the Date reference:
A specific point in time, independent of any calendar or time zone.
...
A Date value encapsulate a single point in time, independent of any particular calendrical system or time zone. Date values represent a time interval relative to an absolute reference date.
So there is no such thing as “an EST date object” in the iOS SDK.
Time zones are relevant when converting to or from strings and when manipulating the calendrical components (year, month, day, hour, minute, second, etc.) of a Date.
The code you posted computes a new Date that is offset from the original date by the time offset between GMT and EST, but the new Date is not “an EST date object”.
(Incidentally, "EST" means Eastern Standard Time, and will not switch to daylight saving time during the appropriate part of the year. Use "US/Eastern" to get the appropriate time conversion depending on the day of year.)
You say you're doing this adjustment “to do some time comparison”, but you don't say what you're comparing to. It would help for you to describe what comparison you're performing.
For example, let's you want to check whether the Date is in the range 8:30 AM to 3:00 PM in the US/Eastern time zone, regardless of the day. If the input string is GMT, then you should parse it that way, as you do in your code:
let parser = DateFormatter()
parser.timeZone = TimeZone.init(identifier: "GMT")
parser.dateFormat = "yyyy-MM-dd HH:mm"
let date = parser.date(from: "2021-03-26 19:55")!
Then, use a Calendar set to the "US/Eastern" TimeZone to compute the hour and minute components of the Date:
var calendar = Calendar.current
calendar.timeZone = TimeZone(identifier: "US/Eastern")!
let components = calendar.dateComponents([.hour, .minute], from: date)
let hhmm = components.hour! * 100 + components.minute!
if (08_30 ..< 15_00).contains(hhmm) {
print("school time")
} else {
print("play time")
}