You can set a simple timer, display it at the top of the screen, and extend/shorten the remaining time.
Commands:
/settimer <time>
Set timer.
/addtimer <time>
increase remaining time.
/removetimer <time>
reduce remaining time.
/canceltimer
Cancel timer.

Time can be specified using h,m,s.
If you enter only a number, the units are seconds.
examples:
1m
1h30m
10s
1m20s
30