IMHO, being half a second off doesn't matter in this situation (measuring "system data"). But if the situation were different (scientific data) a fraction of a second could be very important.
In this situation the main thing that needs to be avoided is drift. If you want 60 second intervals, but you have 60.5 second intervals, those 0.5 seconds will add up and you will creep away from where you want to be. In an hour, it'll be off by 30 seconds, etc... What you need is some way of compensating (see all the good solutions above) so that the 0.5 second error doesn't accumulate.