time since the epoch

[Peter Thiemann]

>>>>> "Stefan" =3D=3D Stefan Karrmann <sk@mathematik.uni-ulm.de> writes:

    Stefan> Peter Thiemann (Thu, Feb 06, 2003 at 12:40:14PM -0800):
    >> John's code illustrates TimeDiff's deficiencies perfectly:
    >>=20
    >> There is also a more fundamental problem with the TimeDiff data
    >> type. While seconds, minutes, hours, and days are clearly specified
    >> amounts of time, the duration of a month or a year may vary depending
    >> on the reference point where the time difference is applied.

    Stefan> What time takes a year - 365 or 366 days? There are leap years!
    Stefan> What time takes a month - 28, 29, 30 or 31 days?
    Stefan> A week takes 7 days.
    Stefan> How long is a day - 86399, 86400 or 86401 seconds and how long =
is a
    Stefan> hour - 3599, 3600 or 3601 seconds? There are leap seconds!
    Stefan> A second is the basic amount of time.

Minutes, hours, and days (even weeks) are also well defined in terms
of seconds. But not months and years.

    >> My conclusion is that time differences really should be measured in
    >> seconds and picoseconds.=20

    Stefan> How do you measure 100 attoseconds?

    >> type TimeDiff =3D (Integer, Integer)

    Stefan> More general is

    Stefan> newtype TimeDiff =3D TimeDiff Rational
    Stefan>    deriving (Eq, Ord)

I agree, I just was not bold enough to propose this :-)
However, this seems to be close to the limit of the measurable, and
I'm wondering how much precision is required in practice.

    >> Hmm, this is underspecified!
    >> As another poster said, (pointing out http://cr.yp.to/libtai, but it
    >> is better to look at http://cr.yp.to/time.html, which has a discussi=
on
    >> on UTC vs TAI vs UNIX time) the official source of time is TAI, so it
    >> is best to base a time library
    >> *on the number of TAI seconds since a reference date*
    >> (which is btw what the libtai is all about).
    >> For compatibility with UNIX time, "Arthur David Olson's popular time
    >> library uses an epoch of 1970-01-01 00:00:10 TAI"
    >> [http://cr.yp.to/proto/utctai.html].=20
    >> So this mostly means that you need to set your system clock correctl=
y:-)

    Stefan> No, you have to check for leap seconds. There is one in
    Stefan> every few years.=20

I do not understand this comment. All I am saying is that you should
set your system clock to the number of seconds since the epoch. Leap
seconds only come into play when converting to UTC.

--=20
Peter Thiemann, Prof. Dr.
Institut f=FCr Informatik, Universit=E4t Freiburg, Germany
http://www.informatik.uni-freiburg.de/~thiemann