>>>>> "Stefan" == Stefan Karrmann <[EMAIL PROTECTED]> writes:
Stefan> Peter Thiemann (Thu, Feb 06, 2003 at 12:40:14PM -0800):
>> John's code illustrates TimeDiff's deficiencies perfectly:
>>
>> 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.
Stefan> How do you measure 100 attoseconds?
>> type TimeDiff = (Integer, Integer)
Stefan> More general is
Stefan> newtype TimeDiff = 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 discussion
>> 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].
>> So this mostly means that you need to set your system clock correctly:-)
Stefan> No, you have to check for leap seconds. There is one in
Stefan> every few years.
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.
--
Peter Thiemann, Prof. Dr.
Institut für Informatik, Universität Freiburg, Germany
http://www.informatik.uni-freiburg.de/~thiemann
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