What you mean the 24 hour clock
00:00 - Midnight
01:00 - 1AM
02:00 - 2 AM
03:00 - 3 AM
04:00 - 4AM
05:00 - 5AM
06:00 - 6AM
....
12:00 - 12 Noon
13:00 - 1PM
14:00 - 2PM
15:00 - 3PM
16:00 - 4PM
...
23:00 - 11PM
then back to 00:00
Basically if you are thinking of a PM time just add the number 12 to the hour... so 4PM would be 16:00... because 4 plus 12 equals 16