What's the Difference Between 'kk:mm', 'HH:mm', and 'hh:mm' in SimpleDateFormat for Time Representation?

Understanding the Differences in SimpleDateFormat Hour Formats: kk:mm, HH:mm, and hh:mm

Various formats exist in SimpleDateFormat to represent time, including kk:mm, HH:mm, and hh:mm. Each format serves a specific purpose.

kk:mm (01-24 hour format):

This format represents time using a 01 to 24-hour clock, with 01 indicating 1:00 AM and 24 indicating midnight. This format is commonly used in military notation and in some European countries.

HH:mm (00-23 hour format):

This format uses a 00 to 23-hour clock, with 00 indicating midnight and 23 indicating 11:00 PM. It's the standard 24-hour format used in most parts of the world.

hh:mm (12-hour format with AM/PM):

This format follows a 12-hour clock, with hh representing hours (01 to 12) and mm representing minutes. AM or PM is used to indicate whether the time is before or after noon.

Example:

The following Java code snippet demonstrates the differences between these formats:

SimpleDateFormat broken = new SimpleDateFormat("kk:mm:ss");
broken.setTimeZone(TimeZone.getTimeZone("Etc/UTC"));
SimpleDateFormat working = new SimpleDateFormat("HH:mm:ss");
working.setTimeZone(TimeZone.getTimeZone("Etc/UTC"));
SimpleDateFormat working2 = new SimpleDateFormat("hh:mm:ss");
working.setTimeZone(TimeZone.getTimeZone("Etc/UTC"));

Date epoch = new Date(0); // Represents 1970-01-01 00:00:00 UTC

System.out.println(broken.format(epoch));
System.out.println(working.format(epoch));
System.out.println(working2.format(epoch));

Output:

24:00:00
00:00:00
05:30:00

As you can see, "kk:mm" prints the time in military format, "HH:mm" prints in standard 24-hour format, and "hh:mm" prints in 12-hour format with AM/PM notation.

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