Well, I looked at the code for the time conditions page and the bad news is that it's hard-coded this way.  The good news is that it shouldn't be all THAT difficult for you to change, if you want to give it a try.  It's in app/time_conditions/ and the file is time_condition_edit.php.  Make a backup copy first, but then you can search for:
case 'time-of-day'
In the block below that, you'll need to change the two lines starting with sel_start.options and sel_stop.options.
You can change these to:
sel_start.options[sel_start.options.length] = new Option(pad(h, 2) + ':' + pad(m, 2));
sel_stop.options[sel_stop.options.length] = new Option(pad(h, 2) + ':' + pad(m, 2));
That should accomplish what you want.