For global game timers you should probably always use global variables.
Just set two variables; GameStartTime and GameTime
Then on start of layout set GameStartTime to System.time
and from then on every tick calculate GameTime = System.time - GameStartTime
If you are counting down from 125 seconds, then GameTime would be 125 - ( System.time - GameStartTime ) and you get a countdown from 125 to 0.
Now to add or deduct 5 seconds, you can use another variable instead of 125, set it to 125 and then add or subtract 5 as needed from that, or you could +- GameStartTime variable as well and keep the calculation the same 125 - ( System.time - GameStartTime )