12 hours from a certain time is just changing the p.m./a.m. to the opposite abbreviation. Since this is 13 hours, we just add one hour to opposite which would be 4 am.
If somewhere is 12 hours ahead/behind another place you would basically just be changing the sign behind the number, get it? So in this case, it's 13 hours so now we just add a number to the time, soo it would be 4am. Hope this helps :)
