greedyisg00d: Sorry, I thought it would be obvious that my second reply was a follow-up to my first reply. They are both usable only with 5.3 and up, as indicated in the manual entry for DateTime::diff().
bpat1434: I assume you meant that the method I used, DateTime::diff() (and the object type it returns, DateInterval), is available only as of 5.3. The DateTime class itself, along with most of its methods, is available with 5.2.0 and up; diff(), add(), sub(), and a couple other methods were added in 5.3.