Got a place you need to be at 3:00? Want to send yourself a message to get out the door at 2:40? Terminal's "leave" command offers a simple way to remind yourself about your upcoming schedule. leave 0240 waits until 2:35 and then alerts you to get ready to leave with both text and a beep. Reminders occur at 5 minutes and 1 minutes before the time you enter, and then every minute after until you close the terminal window. (You can also kill the process whose id is listed for you when you issue the leave command.)
Leave uses a 12-hour clock so you don't have to worry about whether to use 0240 or 1440. Both produce the same result. All times are assumed to be within the next 12 hours.
You can also use the "+" flag to set a relative time. Say you want to work on a project for just the next hour. Use leave +0100. This sets an alarm for one hour from the current time.