In Fiddler, the Caching tab will attempt to calculate the cache freshness lifetime for responses that lack an explicit Expires or Cache-Control: max-age directive. The standard suggests clients use (0.1 * (DateTime.Now – Last-Modified)) as a heuristic freshness lifetime.
An alert Fiddler user noticed that the values he was seeing were slightly off what he expected: sometimes the values were 6 minutes shorter than he thought they should be.
Consider the following scenarios:
Date: February 29, 2016 01:00:00
Date: March 14, 2016 01:00:00
Date: November 7, 01:00:00
Due to the “fall back” adjustment of Daylight Savings Time, these values are 25 hours apart (1500 minutes); 10% of that is 150 minutes.
So when a timespan encompasses an even number of those DST transitions, the effect cancels out. When a timespan encompasses an odd number of these DST transitions, the span is either an hour longer or an hour shorter than it would be if Daylight Savings Time did not exist.
-Eric
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