You don't check 12:59, you check 13:00... if you clock is a 12h clock you count until 12:59, add another minut, your minutes count reset and adds 1 to the hour, your program check that 13 isn't ok to display and instead of displaying 13:00, you display 1:00.

If you have a clock then you should make sure that you can adjust the time so it displays the correct time. If you can't adjust the time it is a timer, that shows how long has been elapsed since it was turned on.

Look older messages, I posted a link for a code that I wrote of a timer without interrupts.

By the way, if your clock is a 12h type I think that you should have some sign that indicates if it AM or PM time, so instead a 12h it would be easier a 24h, that at 23:59 would reset to 00:00 in the next second.

hth

orlando