You dont need to call an sm_cvar as it is quite similar to rcon, and it makes you assume the server has source mod. All you have to do is enter a Command <string> eg from a trigger_once OnStartTouch - Command - Command - mp_timelimit 25 which will change the servers map time.
Warning people (the ones that upload your map) usually dont like when you mess with their server through your map.
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