Help with determinig open and close of day

I live in london (GMT). With respect to ploting pivot points, If today is 12/03/2007 16:00 hrs (GMT), from which bar should i pick my values, is it the 11/03/2007 00:00 hrs bar from the daily chart??

Go with GMT or 1700 EST + 4 hours? when the east coast is on Day Light saving time like now then it is +4. Does the UK have DST? when normal it is +5 hours or whatever your charts say on the daily or the OHLC bars.
Any luck with finding support and resistance without the calculations?