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.