zulip/templates/zerver/help/change-the-date-and-time-format.md

Change the date and time format

By default, messages in Zulip are displayed with a 12-hour timestamp (e.g. 3:00 PM, not 15:00). If you prefer to see the time displayed in a 24-hour format, you can do so easily using the following procedure:

{settings_tab|display-settings}

2. Select the option labeled 24-hour time (17:00 instead of 5:00 PM).