** multiples of 30 minutes **
You may be able to change the minutes textbox form field in the schedule_event_non_recur.html and schedule_event_recurring.html admin templates to a select form field with options of only "00" and "30". Right now those fields look like:
<INPUT TYPE="text" NAME="SCHEDULE_end_time_mm" SIZE="2" MAXLENGTH="2" VALUE="<%=$_end_mm%>" onFocus="" onChange="return onChangeTime(this,'mm');">
You'd likely have to change the JavaScript validation routine to handle the field type changes as well.
Dan O.
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