If you initially set due dates to something like 11 pm PDT on a date like December 1, then when you hit save, your time was automatically translated. Since the dominant time zone on Dec 1 is PST, it was translated to 10pm PST (because 11pm PDT = 10pm PST). This happened way back when you set due dates, not when the daylight savings switch happened.
One solution is to not specify time zone when initially setting due dates.