casemod:
Since this is a sticky it's worth to post this method here as well:
Added 02 Oct 2014 --- @casemod seems to have moved his posts out of this Thread (as was suggested in the next Post) hence this and some of the following posts are in a bit of a vacuum.
Most of the posts in this Thread are about the cumulative error associated with
previousmillis = currentmillis;
and how it is better to use
previousmillis += time;
or (in your second method)
since += time
Please don't confuse newcomers by recommending the deprecated approach in this Thread.
It would probably also be very useful for newcomers if you would modify your Post and add an explanation about how &since works. Most of them will find that concept very confusing without a clear explanation.
...R