Daniel Cousineau has written up some timely advice in a new post to his blog. It looks at pinpointing the time boundary for Daylight Savings Time in a PHP script (when it starts and when it ends).

I had an issue recently where I needed to calculate the Unix timestamp for the daylight savings time boundaries. According to the United States Naval Observatory, daylight savings time begins the Second Sunday of March and ends on the First Sunday of November.

He looks at using the strtotime function to calculate these dates but points out some quirks - like what happens when you just give it a month or something like "second Sunday". His solution was to go back one day ("March 0" instead of "March 1") and calculating the time from there, including that first full day of March in the calculation.