Executing code on, or after a certain time/date?

I'm looking for a way to execute an app, or a block of code at or after a certain time. I've used cron and crontab in the terminal to arrange certain programs to run weekly at the certain date/time. However, if the computer is off at the time specified, the program doesn't run at all. What I need to do in this new application is have a program run at a certain date/time if the computer is on, or start running when the computer is turned on for the first time after that date/time has passed. Any ideas?

 

Thanks, that launchd would work if the computer was asleep at the scheduled time, but it says here:

https://developer.apple.com/library...bs.html#//apple_ref/doc/uid/10000172i-CH1-SW2

That if the computer is off during the scheduled time, the program won't run until the next scheduled time. That's not much better than cron.

 

Thanks Lee, I'll take a look into the "at" command.

Max, do items set as startup items run when you awake from sleep, when you log in, or just when you turn on the computer?