[Datetime-SIG] Computing .dst() as a timedelta

Tim Peters tim.peters at gmail.com
Mon Sep 21 18:04:21 CEST 2015

[Marc-Andre, on Antarctica/Troll]
> Looks like assigning a "time zone" to the place is simply conceptually
> wrong and was just done to make some tz folks happy.

There are many "time zones" in Antarctica (theoretically, it's in all
time zones).  They're all senseless ;-)


> Anyway, the main takeaway for me is that it is obviously possible
> to have more than two DST switches during the year, which is
> something I wasn't aware of before seeing this example.

The Brits beat 'em to it, but a long time ago:


    In 1940, during the Second World War, the clocks in Britain
    were not put back by an hour at the end of Summer Time.
    In subsequent years, clocks continued to be advanced by
    one hour each spring and put back by an hour each autumn
    until July 1945. During these summers, therefore, Britain was
    two hours ahead of GMT and operating on British Double
    Summer Time (BDST). The clocks were brought back in line
    with GMT at the end of summer in 1945. In 1947, due to
    severe fuel shortages, clocks were advanced by one hour
    on two occasions during the spring, and put back by one hour
    on two occasions during the autumn, meaning that Britain was
    back on BDST during that summer.

These may be the corresponding lines in IANA's "europe" file:

Rule GB-Eire 1947 only - Mar 16 2:00s 1:00 BST
Rule GB-Eire 1947 only - Apr 13 1:00s 2:00 BDST
Rule GB-Eire 1947 only - Aug 10 1:00s 1:00 BST
Rule GB-Eire 1947 only - Nov 2 2:00s 0 GMT

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