T-baneklokke - coundown clock

Some time ago I grew tired of my wife asking if she had to leave soon to get to the metro in time. I did what any sensible techie would and made her a battery powered countdown clock. The clock counts down to a selectable time, and repeats the countdown every 15 minutes (the normal interval for the metro over here). Three minutes before it is time to leave, the clock display starts flashing slowly to get your attention. Then, once you should have left, the clock counts UP, blinking fast, as you still have time to make it if you walk briskly. Once your time is TRULY up the countdown to the next metro starts.”