This is my personally preferred method for creating a countdown timer in Arma 3.

initCountdownTimer.sqf

//*** Initialize the countdown timer for all players
sv_currentTime = 5*60;
[ "itemAdd", [ 
	"countdownTimer", {		
		sv_currentTime = sv_currentTime - 1;
		if (sv_currentTime isEqualTo 0) then {
			sv_currentTime = nil;
			"EveryoneLost" call BIS_fnc_endMissionServer;
		};
	}, 
	1, "seconds", {
		!isNil "sv_currentTime" && {sv_currentTime >= 0}
	}
] ] call BIS_fnc_loop; 



//*** Here's how I like to display the timer (via hint message):
sxf_fnc_loopMessage = {	
	_output = "";
	_timerMessage = "";	
	
	if (!isNil "sv_currentTime") then {
		_timerMessage = _timerMessage + "<t size='1.3'>Time remaining:</t><br/>";
		_timerMessage = _timerMessage + format["<t size='2' align='center' color='#F1BD1D'>%1m %2s</t>", floor (sv_currentTime/60), sv_currentTime%60];
	};	
	
	_output = (
		"<br/><t align='left'>" +
		_timerMessage +
		"<br/><br/></t>"
	);
	
	{	//show the mission info hint message only to the players who have their loop message turned ON
		if (_x getVariable ["sxf_bLoopEnabled", false]) then {
			(parseText _output) remoteExec ["hintSilent", _x];
		};
	} forEach (call BIS_fnc_listPlayers);
};
[ "itemAdd", [ "loopMessage", { call sxf_fnc_loopMessage; }, 1, "seconds", {
  { _x getVariable ["sxf_bLoopEnabled", false] } count (call BIS_fnc_listPlayers) > 0  
} ] ] call BIS_fnc_loop;



//*** Allow the players to toggle the looping hint message described above:
//(move everything below this line to initPlayerServer.sqf if you want it to work for JIPs, I guess
{!(IsNull (findDisplay 46))} remoteExec ["waitUntil"];
[(findDisplay 46), "KeyDown"] remoteExec ["displayRemoveAllEventHandlers"];
[
	(findDisplay 46),
	[
		"KeyDown",
		{
			if (!isDedicated && {!isNil "sxf_fnc_handleKey_F1"}) then {
				sxf_fnc_handleKey_F1 = {
					_temp = player getVariable ['sxf_bLoopEnabled', false];
					if (_temp) then {hintSilent '';};
					playSound ( ['AddItemFailed', 'AddItemOK'] select !_temp );
					player setVariable ['sxf_bLoopEnabled', !_temp, true];
				};
			};
			if ( (_this select 1) in ( (actionKeys 'User1') + [0x3b] ) ) then {call sxf_fnc_handleKey_F1;};
		}
	]
] remoteExec ["displayAddEventHandler"];