I can back it up. A.m./p.m. is only used in the English speaking world (and some former British/US colonies, I'd assume). The rest of the world use ISO 8601-compliant time,
You'd especially get some problems if you need to schedule something at eg. 12:30 a.m./p.m. A lot of those who do 'get' the a.m./p.m. system will just add 12 to the p.m. time, and will be confused when they come across 12:30 p.m. This will then be seen as 24:30, which would be understood as '30 minutes past midnight the following day'.
It'd be better if the US and UK would just catch up with the rest of the world