Whether one uses the 12 hour system or the 24 hour system, here's a handy tip for quick conversion.
Having grown up using a 12 hour clock and the Imperial measuring system, I'm always on the lookout for easy ways to convert one thing to another - Imperial to Metric and back again. Preferably without having to do much math.
The Alien Eyes blog has a great tip for converting time.
I cheerfully ignored the comments that implied everyone should have learned this from their (insert grade or level) math teacher, feel free to do the same.
Check it out here: