The time of day is in the form hhmm where hh is a time in hours (on a 12 or 24 hour clock), and mm are minutes. All times are converted to a 12 hour clock, and assumed to be in the next 12 hours.
If the time is preceded by '+', the alarm will go off in hours and minutes from the current time.
If no argument is given, leave prompts with "When do you have to leave?". A reply of newline causes leave to exit, otherwise the reply is assumed to be a time. This form is suitable for inclusion in a .login or .profile.
To get rid of leave you should either log off or use kill -s KILL giving its process id.