Type for Current Time Function

[Bayley, Alistair]

> From: Simon Marlow [mailto:simonmar at microsoft.com] 
> 
> I think I don't really understand the (days,ticks) representation.
> Could somebody explain the meaning of arithmetic on this type?  
> 
> Is the ticks field supposed to be limited to a day's worth?  
> Or a day + a leap second?


I'm assuming that it's UTC (i.e. ticks = day plus optional leap second),
although others might think differently...


> Can I add ticks to a time?  If so, don't you need leap second 
> info to do
> the normalisation?  Or, if normalisation doesn't happen, you need leap
> second info to convert to UTC, right?  The only conclusion seems to be
> that either you can't add ticks to a time, or you need leap 
> second info
> which is what we're trying to avoid.

Isn't this a general problem, rather than a specific representation problem?
The only representation where you wouldn't have to know about leap seconds
would be TAI.

E.g. there was a leap second on 1997-06-30 23:59:60 UTC, so:

 - Q: how many seconds are there between 1997-06-30 00:00:00 UTC and
1997-07-01 00:00:00 UTC ?
   A: 86401

 - Q: how many days are there between 1997-06-30 00:00:00 UTC and 1997-07-01
00:00:00 UTC ?
   A: 1

 - Q: how long is the period between 1997-06-30 00:00:00 UTC and 1997-07-01
00:00:00 UTC ?
   A: 1 day 1 second, or 1 day ? (I guess just 1 day, as a day can be 86401
seconds long.)

 - Q: how many seconds in a day?
   A: 86400 (or maybe this is undefined, as it could be 86399, 86400,
86401?)

 - Q: what is 1997-06-30 00:00:00 UTC plus one day?
   A: 1997-07-01 00:00:00 UTC

 - Q: what is 1997-06-30 00:00:00 UTC plus 86400 seconds?
   A: 1997-06-30 23:59:60 UTC


It gets hairy when you start thinking about durations, and I would like to
see duration arithmetic in the library. I don't have any good ideas about
how to handle this.


Alistair.

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