code to allow for daylight settings

I am looking at code to extrapolate current data into the future to predict
events (obviously this cannot be an exact science!)
Estimated event time will be given as date and time of day
I thought of the scenario where the extrapolation spans a daylight settings
time change (eg: Sun 25-Oct-09 01:00) clocks go back 1 hour in UK.
With an extra hour in the calendar I would like to adjust my event
prediction accordingly.
No problem to write code using specific times and dates, but I wondered if
there is a way in code to detect system daylight settings (ie:GMT, GMT + 1,
etc) and use this as criteria for adjustment?
Any advice will be appreciated