in case anyone needs it, for years I calculated 24 hour time by adding 1 to the first digit and 2 to the second, and leaving the last two. Except it's more "whatever makes the easiest math"
0900pm becomes 9+2+10.00 2100h, 1159pm becomes 11+10+2.59 2359h, 1240am becomes 2440h, or 0040h depending on which day it is.
I mostly learned it for better timezone math on the fly when playing EVE Online, but i recommend having better, more compelling reasons
