[Python-Dev] RE: [Python-checkins] CVS: python/nondist/sandbox/datetime datetime.py,1.50,1.51

[Tim Peters]

[MAL, on DST for out-of-platform-mktime()-range years]
> My point was to not fiddle with the date at all: raise an
> exception and that's it.

If someone wants to print some datetime object from the year 1537 as =
if a
local time, raising an exception just because the local DST rules in =
effect
at that time are unknown seems extremely unfriendly.  For goodness sa=
ke,
we're already "lying" to them about the calendar system in effect at =
that
time, and an hour more or less due to DST uncertainty is trivial.  It=
's an
idealized calendar, and an idealized time.

We should supply a way for the user to find out whether or not a DST
correction was applied, though.  ECMAScript does that via a

    DaylightSavingTA(t)

function mapping UTC time to the number of milliseconds (all their ti=
mes are
in milliseconds) to be added to t to adjust for local DST.  A distinc=
t
LocalTZA constant gives the number of msecs to add to UTC to adjust f=
or
local timezone.

    Conversion from UTC to local time is defined by

        LocalTime(t) =3D t + LocalTZA + DaylightSavingTA(t)

    Conversion from local time to UTC is defined by

        UTC(t) =3D t =96 LocalTZA =96 DaylightSavingTA(t =96 LocalTZA=
)

    Note that UTC(LocalTime(t)) is not necessarily always equal to t.

That last line gives me an especially warm feeling <wink>.

there's-idealized-and-then-there's-idealized-ly y'rs  - tim