DST Asymmetry Reminder
It's April: time for my annual reminder about the asymmetry of
daylight savings time. If you've seen this before, well, here's
The summary: Daylight Savings Time is not symmetrical around the
solar calendar -- it does not "surround" the days of longest light
as one might expect if one hasn't thought about it. So if you like
to do outdoors stuff (e.g., hiking or mountain-bike riding) while
it's light out, especially mid-week after work, the time to get
started with that sort of outdoors stuff is NOW.
Some details: If we were to start DST at the spring equinox and end
it at the autumn equinox, then it would be close to symmetric in the
sense I'm talking about. Around the day we spring ahead into
daylight savings time, sunset would change from being (say) 6:30 to
being 7:30; and the day we fall back to winter time, it would go from
7:30 to 6:30. But we don't do the DST changes until some time after
the equinoxes. We spring ahead in early April, roughly 11 to 18 days
after the spring equinox, and we fall back in early October,
somewhere around two weeks after the fall equinox. This means that
it's lighter later into each day at the start of DST than it is at
the end. In fact, a little bit of trigonometry (or some actual
observation) will tell you that the equinoxes are the times of the
fastest change in the days' lengths, so it's lighter a lot later at
the start of DST than at the end.
Around here (San Francisco Bay Area), the springtime change this
year made our notion of sunset change from 6:30pm to 7:30pm. So
it's already worth getting out for a short (~90-minute) hike or
ride after work even if you have to work until 6:00pm.
By contrast, the autumn change around here usually seems to make our
label for "sunset-time" change from about 6:00pm to 5:00pm or so.
That's an hour and a half difference, compared to the springtime
change. It generally means that despite the usually dry autumn
weather we get, those of us who have to work until 6:00pm or later
have few choices by October -- learn to like riding with lights
(and to find places where it's legal), or stick to riding or hiking
only on weekends, "hookey" weekdays, or long "lunch"es.
Bottom line: For those of us for whom snow is not an issue, the dry
parts of April are the time to start those after work hikes or rides
(except for El Niño years, whose Aprils have no dry parts).