You could turn on Flag 1 and have another event (Continuous?) that increments this Flag 1 -- that will run every second and should provide an elapsed time indication.
About a quarter way down in the WC's mist examples doc there is this that uses a timer --
--- old way
local units = mist.makeUnitTable({'RDR AGrp01-1'})
local flag = 10117
local zones = {'Gudauta Zone'}
local zone_type = 'cylinder'
local function unitsInZonesOnOff(result)
local ret = mist.getUnitsInZones(units, zones, zone_type)
if #ret == 0 and result == true then
trigger.action.setUserFlag(flag, 0)
result = false
elseif #ret > 0 and result == false then
trigger.action.setUserFlag(flag, 1)
result = true
end
mist.scheduleFunction (unitsInZonesOnOff, {result}, timer.getTime() + 1)
end
unitsInZonesOnOff(false)
--- ^^^ old way
I'm not an expert on this stuff... but I hope this helps.
WC