You can specify 12 hour or 24 hour clocks in your time formats. If you enter a single time format, times will be shown as the 24 hour clock. If you enter two formats, separated by a semi-colon (and no spaces) the 12 hour clock will be used. So
hh:mm:ss
will produce 24 hour numbers and
hh:mm:ss;hh:mm:ss
will produce 12 hour numbers. If you create 12 hour clocks, you should also use a text string in the format to indicate whether times are am or pm.