What are the differences between HH:mm, hh:mm, and kk:mm in Java's SimpleDateFormat?

Distinguishing between java Formatting Patterns: HH:mm, hh:mm, and kk:mm

The SimpleDateFormat class in Java provides various format patterns to display dates and times. While the HH:mm, hh:mm, and kk:mm patterns may seem similar, there are notable differences in their representations.

Consider the following Java code snippet:

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"));

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

The output of the provided code is:

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

Let's break down the differences between these patterns:

• kk:mm: Represents hours (01-24) in a 24-hour format. The output "24:00:00" indicates midnight in this format.
• HH:mm: Represents hours (00-23) in a 24-hour format. The output "00:00:00" denotes 12:00 AM in this pattern.
• hh:mm: Represents hours (01-12) using a 12-hour format with AM/PM designations. However, it's important to note that the code example does not set the format to "hh:mm:ss". Instead, it uses the same format as "working", which is "HH:mm:ss". As a result, the output "05:30:00" is incorrect and should have been "12:00:00" in a 12-hour AM/PM format.

Therefore, understanding the nuances of these formatting patterns is crucial for accurately representing dates and times in Java.

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