It seems that the web interface cannot accommodate entering timestamps in 24-hour format (military time). Having been a systems administrator for a long time I find the 24-hour timestamps much easier to handle when dealing with schedules. (Makes the time math easier.)
It would be great if the web site handled the time format in a manner simliar to how it handles time zones. Have an account setting that gives the time format for the entire site, then allow the users to change the format for their own use.
This is also an internationalization / localization issue, in that entire provinces in Canada use 24-hour time instead of 12-hour AM/PM clocks. (Thinking of Quebec here.)