How to change the time format (12/24 hours) of an <input>?

HTML5 Time Input

This one is the simplest of the date/time related types, allowing the user to select a time on a 24/12 hour clock, usually depending on the user's OS locale configuration. The value returned is in 24h hours:minutes format, which will look something like 14:30.

More details, including the appearance for each browser, can be found on MDN.

<input type="time" name="time">

Its depends on your locale system time settings, make 24 hours then it will show you 24 hours time.

By HTML5 drafts, input type=time creates a control for time of the day input, expected to be implemented using “the user’s preferred presentation”. But this really means using a widget where time presentation follows the rules of the browser’s locale. So independently of the language of the surrounding content, the presentation varies by the language of the browser, the language of the underlying operating system, or the system-wide locale settings (depending on browser). For example, using a Finnish-language version of Chrome, I see the widget as using the standard 24-hour clock. Your mileage will vary.

Thus, input type=time are based on an idea of localization that takes it all out of the hands of the page author. This is intentional; the problem has been raised in HTML5 discussions several times, with the same outcome: no change. (Except possibly added clarifications to the text, making this behavior described as intended.)

Note that pattern and placeholder attributes are not allowed in input type=time. And placeholder="hrs:mins", if it were implemented, would be potentially misleading. It’s quite possible that the user has to type 12.30 (with a period) and not 12:30, when the browser locale uses “.” as a separator in times.

My conclusion is that you should use input type=text, with pattern attribute and with some JavaScript that checks the input for correctness on browsers that do not support the pattern attribute natively.