Thanks for the recommendation Aphrodite - I think there's also a performance issue here. For example, clocking something every second (or ten times per second for slightly more accuracy) is using a lot more resource than doing it just once per layout.
Performances would not be that much hurt I would think, but well, if we can prevent doing a redondant thing by using something the engine actually handle itself, why not do it.(I am more thinking of tool usage rather than performances, as using the tools we are given wisely is actually, from my experience, giving good performances).
You also have the advantage of putting the calcul in a function if you want to, that would return the spent time in second relative to the initial time for exemple, which may make the code clearer to an extend.