Well, my first guess is 132 for 24 hour time format.

You can only use the numbers which look the same or like another number in mirror image, so 0,1,2,5 and 8 but 8 is a problem because you can't go over the 24 hour limit and for the same reason the 2 on the end of a time e.g. 11:02 cannot be used for the times over 24 hours because it would reverse to 50:11.

(Do I have to reveal that I am an MsExcel nerd, or can I just let you think I did it in my head?)