Most Unix-variants use the
Olson time zone database and time library, which is able to model time zone rules that change over time. The Olson time system knows what the DST rules used to be, what they are now, and what they will be in the future (that is, if anyone knows, and if the TZ database has been updated with that information.)
MacOS X, since it's a Unix variant, uses the Olson time library and TZ data base. The new US DST rules were passed by Congress in August 2005, and were incorporated into the TZ database within days of the law taking effect. So those new DST rules have been in MacOS X for over two years now--more than a year before the first DST transition happened according to the new rules (from 1987 through 2006, the rules used to be first Sunday in April, last Sunday in October, but since 2007 the transition to DST happens on the second Sunday in March, and the transition back to standard time happens on the first Sunday in November.)
Note that the new DST rules were in the TZ database during all of 2006, but were not applied to dates/time prior to 2007. Note also that the old rules will be applied to timestamps prior to 2007, and even older rules will be applied to timestamps prior to 1987.
So worry about Windows, not about MacOS X.
