Use 2pm, 3.30am (closed up) unless there is a good reason not to.
Note: noon is 12pm, midnight is 12am. It may be clearer to say "noon" or "midnight"
For press releases as well as military, space and similar stories, the 24-hour clock may be more appropriate:
1630 GMT, 1130 Eastern Standard Time
Note: To configure the correct date format on a Drupal site, go to admin/config/regional/date-time/formats/40/edit.
The PHP code you need to add in the Format string box is "j M Y, g.ia"