You could probably do this with a shell script and run it using cron or Alarmed. But the real solution is to plan to leave 5/10 minutes earlier. I do know what it is like, I'm just the same, leaving late does get me running more though

How would you work this though? Would you just have it run once, but then you'd get used to the offset after a while, or would you run it every night, but the the offset would add up and after a week it could be anywhere between 7 and 70 minutes fast. I guess the best way would be for it to check the time via the internet and do a random offset each day based on that.