I have already done this for a french server.
You need an admin to launch the timer (with rcon), and this is a fun plugin.
Example :
- At the start, an admin write : rcon sm_pbchrono 360 (360 seconds = 6 minuts).
- So, survivors have 6 minuts to finish their round.
- If a survivor die, they lose time.
- If the time runs out, survivors run slowly, and a special horde will be called (they become fast and hard to be killed).
This is a fun plugin.
If you are interested, pm me.