iOS DateFormatter - Format Time in shortest way possible

How about creating the formatter yourself?

First you retrieve the hour and the minute from your date. Then you check if minute is 0.

extension Date {
    private static var componentFlags: Set<Calendar.Component> { [.year, .month, .day, .hour, .minute, .second] }
    
    private func components() -> DateComponents { Calendar.current.dateComponents(Date.componentFlags, from: self) }
    
    func hour() -> Int { return self.components().hour ?? 1 }
    func minute() -> Int { return self.components().minute ?? 1 }
}

let now = Date()
let hour = now.hour()
let strHour = String(format: "%02d", hour)
let minute = now.minute()

let output: String
if minute == 0 {
    output = strHour
} else {
    let strMin = String(format: "%02d", minute)
    output = "\(strHour):\(strMin)"
}

print("For date \(now), output is \(output)")
// OUTPUT:
// For date 2024-04-01 16:00:00 +0000, output is 16
// For date 2024-04-01 16:40:07 +0000, output is 16:40

Note that when you retrieve the hour with this method, the timezone is automatically applied. So if the date is 16:00 UTC+0 and you are in a -6 timezone, you will get 10 from hour(). This is usually the desired behavior.

It appears that your goal is to format a date into a time string showing hours and minutes. But if at the top of the hour (0 minutes), just show the hour.

The basic approach would be to use DateComponents to look at the minute value of the date. Then based on whether it is zero or not, format the date accordingly.

Ideally you could use DateFormatter setLocalizedDateFormatFromTemplate passing in either h:mm or just h or maybe either H:mm or just H. But running some tests with different locales resulted in the wrong localized time format in many cases.

The working solution I found was to set timeStyle to .short and then look at the resulting dateFormat value. When minutes is 0, remove the :mm (or similar) from dateFormat.

Here's some working code:

func shortFormat(date: Date, locale: Locale = Locale.current) -> String {
    // Setup the formatter for localized short time style
    let formatter = DateFormatter()
    formatter.locale = locale
    formatter.dateStyle = .none
    formatter.timeStyle = .short

    // If the date's time is at 0 minutes, remove the minutes from the format
    let mins = Calendar.current.component(.minute, from: date)
    if mins == 0 {
        // Some locales use mm and some use m
        // Locales use a colon, period, or other character
        // The RE '\Wm{1,2}' means a non-word character followed by 1 or 2 m characters.
        if let format = formatter.dateFormat, format.range(of: "\\Wm{1,2}", options: .regularExpression) != nil {
            formatter.dateFormat = format.replacingOccurrences(of: "\\Wm{1,2}", with: "", options: .regularExpression)
        }
    }

    return formatter.string(from: date)
}

Here's some test code:

print(shortFormat(date: date))
print(shortFormat(date: date, locale: Locale(identifier: "de_DE")))
print(shortFormat(date: date, locale: Locale(identifier: "fr_FR")))
print(shortFormat(date: date, locale: Locale(identifier: "es_CL")))

Sample output:

3:54 PM
15:54
15:54
3:54 p. m.

And here's some that ensures 0 minutes:

let now = Date()
var comps = Calendar.current.dateComponents([.calendar, .era, .year, .month, .day, .hour, .second], from: now)
let date = Calendar.current.date(from: comps)!
print(shortFormat(date: date))
print(shortFormat(date: date, locale: Locale(identifier: "de_DE")))
print(shortFormat(date: date, locale: Locale(identifier: "fr_FR")))
print(shortFormat(date: date, locale: Locale(identifier: "es_CL")))

Sample output:

3 PM
15
15
3 p. m.

Note that the output in locales that use 24-hour time may be confusing when minutes is 0. You may wish to consider your approach.