What method are you using to get the time? If you are using the Date plugin, the time is already stored in 24 hour mode and most of the time methods display it that way.
Date.toString(timestamp) returns the date info followed by HH:MM:SS in 24 hr then time zone
Date.toTimeString(timestamp) returns HH:MM:SS in 24 hr followed by time zone information
Date.getHours(timestamp) returns a value between 0 and 23
If you need to get the timestamp, use Date.get(Y,M,D,h,m,s,milli)
Read through the documentation to see more uses:
construct.net/en/make-games/manuals/construct-3/plugin-reference/date
If you aren't using the date object, we would need to know how you are getting the date and time to be able to help you/