Prior to Windows Vista, Windows offered support for only a single set of adjustment rules.


Q: Prior to Windows Vista, Windows offered support for only a single set of adjustment rules. This meant that daylight saving time rules can be applied to any date to generate a historical DST value (regardless if it was accurate). 
A: To date time zone information has been stateless, that is there is only one rule in effect at a time, and ALL dates and times are interpreted by that rule – past, present and future.  So that means if you have applied the 2007 rules to your desktop and go back and look at the third week in March 2006 (assuming US time zone), you will see its DST when in fact it’s not supposed to be.  Dynamic DST will help that in the future.  But today, the OS and all apps interpret local time based on the TZI in the legacy key.  In the future apps will be able to leverage the dynamic keys.

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