NSCalendar does not correct for GMT time

Question

When I compute the start of a day, a time difference of one hour is obtained. Why is not corrected for GMT+1?

let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
cal.timeZone = NSTimeZone.localTimeZone()

let startOfToday = cal.startOfDayForDate(NSDate())

print(NSTimeZone.localTimeZone())
print(startOfToday)

Output:

"Local Time Zone (Europe/Amsterdam (GMT+1) offset 3600)\n"
"2016-01-14 23:00:00 +0000\n"

Answer 1

Your code should work fine, you shouldn't care about time zones. As already mentioned by Martin R "NSDate is an absolute point in time and does not have a timezone". If really need to use UTC time you can set the calendar property to UTC to obtain the startOfDay or noon at UTC time for any date as follow:

extension Calendar {
    static let utc: Calendar  = {
        var calendar = Calendar.current
        calendar.timeZone = TimeZone(identifier: "UTC")!
        return calendar
    }()
    static let localTime: Calendar  = {
        var calendar = Calendar.current
        calendar.timeZone = .current
        return calendar
    }()
}

---

extension Date {
    var noon: Date {
        return Calendar.localTime.date(bySettingHour: 12, minute: 0, second: 0, of: self)!
    }
    var startOfDay: Date {
        return Calendar.localTime.startOfDay(for: self)
    }
    var noonAtUTC: Date {
        return Calendar.utc.date(bySettingHour: 12, minute: 0, second: 0, of: self)!
    }
    var startOfDayAtUTC: Date {
        return Calendar.utc.startOfDay(for: self)
    }
}

---

print(Date().noon)               // "2018-04-30 15:00:00 +0000\n"
print(Date().startOfDay)         // "2018-04-30 03:00:00 +0000\n"

print(Date().noonAtUTC)          // "2018-04-30 12:00:00 +0000\n"
print(Date().startOfDayAtUTC)    // "2018-04-30 00:00:00 +0000\n"