The countdown output is in seconds. But perhaps you did experiment with the units, and found out the time drift caused by the linear sequence of operations. This time drift, although minute, will add up to significant amounts when the timer is set to a high value, like 5 minutes or so.

Here is the output for elapsed time (in milli seconds):

1000
2000
3001
4001
5001
6002
7002
8003
9003
10004