How could i do a rewarding system?

I was thinking if i could get something like the code below, to be datastored and update all the time, even when the player is not in the game. I'm trying to do what Mad Studio did in The Mad Murderer.

time = 43200 --12 hours
while time > 0 do wait(1)
time = time - 1
script.Parent.Value = string.format("d:d:d", math.floor(time/60), time%60)
end

I know i need to use tick() for this type of event, i just need help with this.