Date/Time format is fetched from the user's language.

Enable Developer Mode, then go to Settings -> Translations -> Languages --> select the language in question and find the field Time Format. If you currently have a 12-hour format (am/pm), this field should currently read something like %I:%M:%S. Change it to %H:%M:%S and you should be good to go:

For reference, here is the legend of supported formats (also available directly in the language form):

%a - Abbreviated day of the week.

%A - Full day of the week.

%b - Abbreviated month name.

%B - Full month name."

%d - Day of the month [01,31]."

%j - Day of the year [001,366]."

%H - Hour (24-hour clock) [00,23]."

%I - Hour (12-hour clock) [01,12]."

%M - Minute [00,59]."

%p - Equivalent of either AM or PM."

%S - Seconds [00,61]."

%w - Day of the week number [0(Sunday),6]."

%y - Year without century [00,99]."

%Y - Year with century."

%m - Month number [01,12]."

Gantt view with 12-hour format:

Gantt view with 24-hour format: