Handle roundStartTimer;

public OnPluginStart()
{
    HookEvent("arena_round_start", Event_arena_round_start);
}

public Action Event_arena_round_start(Handle:event, const String:name[], bool:dontBroadcast)
{
	if (roundStartTimer != INVALID_HANDLE)
		KillTimer(roundStartTimer);

    roundStartTimer = CreateTimer(30.0, sometimerfunction,_,TIMER_FLAG_NO_MAPCHANGE);
}

public OnMapEnd()
{
	roundStartTimer=INVALID_HANDLE;
}

public Action sometimerfunction(...)
{
	//do whatever you want here but just make sure roundStartTimer is set to INVALID_HANDLE if you need to return early or something

	roundStartTimer=INVALID_HANDLE;
}