[phobos] datetime review

[Jonathan M Davis]

On Tuesday, October 12, 2010 06:26:16 Michel Fortin wrote:
> I disagree.
> 
> A date can be expressed using different calendars, but can also be stored
> as an interval since a reference date (like the UNIX epoch). With your
> main storage format calendar-agnostic you can easily convert back and
> forth to different calendars-specific formats.

Except that most of the logic is going to be calendar-specific such that you 
really don't gain much by trying to separate out the calendar-specific logic. 
Date, DateTime, and SysTime all have a property to give you the Xth day since 
the beginning of the Proleptic Gregorian Calendar (dayOfGregorianCal IIRC), and 
SysTime has the stdTime property which gives you its internal representation of 
hecto-nanoseconds (100 ns) since midnight January 1st, 1 AD UTC, so all of the 
information necessary to convert to other calendar systems is there. But since 
most of the operations on Date, DateTime, and SysTime are specific to the 
Gregorian Calendar, I don't see much - if any - gain in trying to refactor out 
the non-calendar-specific stuff.

If I do anything to make it easier to handle other calendars, it should be to 
make sure that the duration, interval, and range code is fairly calendar 
agnostic. And for the most part, I think that it is. Having removed 
MonthDuration and JointDuration, it's even more so.

> Also, if you're dealing only with time -- like calculating the time elapsed
> between two time points -- then you don't need any calendar unless you
> plan to display that elapsed time in months or years.
> 
> And of course I never meant that Phobos should provide more calendars than
> gregorian. That said, it'd be good if it its gregorian implementation
> could be used as a base to implement other calendars.

Well, if the duration, interval, and range stuff is reasonably calendar agnostic, 
then the portions that are really of much use to someone wanting to use datetime 
with their own, non-Gregorian Calendar code would be useable with that code. But 
I don't think that it makes much sense to try and make the time point types 
calendar agnostic. The only one that is is TimeOfDay, since it doesn't deal with 
dates at all, and that's quite re-usable in other calendar code.

- Jonathan M Davis