iCal won't allow "custom" time frame

I haven't used iCal for several months due to unemployment. I just opened it to enter the schedule for my new job.

My problem is that once I enter a start time, I am not permitted to enter a custom end time. Instead, I am given a choice of pre-set blocks in half-hour increments. I have used iCal on and off for years and never had this problem. Is it a bug or am I doing something wrong? Thanks for your help.