Create TimeZone object from timeZoneOffset string? - swift

Question

What would be a clean way to initialise a Swift TimeZone object from timeZoneOffset string of the form: "+HH:MM".
I am looking for something of the form:
extension TimeZone {
init?(UTCOffsetString ofs: String) {
let signIndex = ofs.firstIndex(of: "+") ?? ofs.firstIndex(of: "-")
let sign = ofs[signIndex!]
let separatorIndex = ofs.firstIndex(of: ":")!
let hhRange = ofs.index(signIndex!, offsetBy: 1)..<separatorIndex
let hh = ofs[hhRange]
let mmRange = ofs.index(separatorIndex, offsetBy: 1)..<ofs.index(separatorIndex, offsetBy: 3)
let mm = ofs[mmRange]
var offsetInMin = (Int(String(hh))! * 60) + Int(String(mm))!
if sign == "-" {
offsetInMin.negate()
}
let offsetInSec = offsetInMin * 60
// Convert string to TimeZone, eg.
self.init(secondsFromGMT: offsetInSec)
}
}
let tz = TimeZone.init(UTCOffsetString: "-07:30")
print(tz?.identifier ?? "unknown")
The above code block is a correct solution and prints:
GMT-0730
However I am looking for a cleaner solution where I don't need to extract substrings in order to compute the offset.

Answer 1

My suggestion is to use DateFormatter which is able to parse the time zone string format. refZoneString is the reference to UTC in the current time zone.
extension TimeZone {
init?(UTCOffsetString ofs: String) {
let refZoneString = "+0000"
let formatter = DateFormatter()
formatter.dateFormat = "Z"
guard let refDate = formatter.date(from: refZoneString),
let date = formatter.date(from: ofs) else { return nil }
self.init(secondsFromGMT: Calendar.current.dateComponents([.second], from: date, to: refDate).second!)
}
}
let tz = TimeZone.init(UTCOffsetString: "-07:30")
print(tz?.identifier ?? "unknown")

Answer 2

I don't know what you mean by a cleaner but you can combine collection methods suffix and prefix to avoid the need to use String index to access the desired values:
let time = "-02:00"
let hours = Int(time.suffix(5).prefix(2)) ?? 0
let minutes = Int(time.suffix(2)) ?? 0
var offset = hours * 3600 + minutes * 60
if time.first == "-" { offset = -offset }
print(offset) // -7200