The muster and pay rolls are a bit like a school attendance register. They merely note a soldier's presence in order to account for his pay. You can only deduce his date of enlistment from the date he first appears in the rolls, but of course this may not be definitive. Similarly the absence of his name might be for a number of reasons, including desertion, being in prison or in hospital, having been transferred to the other battalion, or of course because he has been discharged.
By the way, I am puzzled by Shaun's reference to the muster rolls for 1818 appearing to be on Ancestry. TNA specifically says the rolls for that year have not been digitised. The TNA entry may be out of date.