Americans use DD/MM/YYYY (regardless of delimiter being e.g. / or - or .). We Dutch use MM/DD/YYYY. The latter results in sorting correct on month if its annual data. Over multiple years it breaks. ISO 8601 has my preference, though in MS Office I need to set to Japanese cause my version lacks the setting.

9.00 PM is very clear to mean 21.00 in 24H format. What isn't clear is if 9.00 is 12H or 24H format. 9.00 AM or 9.00 PM is. However if you may reasonably assume the user uses 24H format, it is clear. It always takes up less space to use 24H format, though 12H format is always clear. Except for TZ (timezone), both all time suffers from that.

[EDIT]Oops I messed up, and that's why I dislike these and prefer ISO8601. Although one nice thing about DD/MM/YYYY is that its little endian (which is easy to remember for laymen as going from small to big). Keeping rest of comment as is.[/EDIT]