std.date proposal

[Fredrik Olsson]

I have created a proposal for a std.date replacement. And announce it 
here in hopes of some comments and criticism.

The parse code in converted from the PostgreSQL C code base, and is 
quite competent. If SQL99 supports it, then PostgreSQL does. Thanks goes 
to the postgres hackers.



The philosophy is that date and time can be represented with four basic 
types:

d_date - For a date with day precision.
d_time - For a time with millisecond precision (No date part).
d_timestamp - For a date+time with better than millisecond precision for
	dates close to present day.
d_duration - A fixed of relative duration of time. A months and a year,
	is relative as no two months are guarantied to have the same
	length, while weeks and hours are fixed (No support for leap
	seconds :) ).

The module is kept simple and consistent, all types are handled with 
just a few functions:

toType() - functions, with many overloads to make a time/date/duration
	from a string, different time units, Unix epoch, and more. For
	example toTime(hour, minute, second) and toDuration(string).

age() - functions with overloads to get the duration between two dates,
	times, or timestamps in fixed or relative units. For example:
	age(toDate("2006-02-01"), toDate("2006-03-01");
	will give "28 days", while:
	age(toDate("2006-02-01"), toDate("2006-03-01", true);
	Will give "1 month".

splitType() - functions with overloads, to split type into it's
	components. For example:
	int year, dayOfYear;
	splitDate(toDate("2006-10-28"), year, dayOfYear);

extract() - With overloads, extract a component of a type, or converts
	the type to special representations. Example:
	extract(toDate("2006-03-28"), DatePart.WEEKDAY);
	will give WeekDay.TUESDAY, and:
	extract(toDate("2006-03-28"), DatePart.EPOCH);
	will give 1143496800.

increment() - With overloads to add both a single date part, and
	duration to each type. For example:
	increment(toTime("01:02:03"), DatePart.HOUR, 4);
	Will give "05:02:03". And:
	increment(toDate("2006-02-12"), toDuration("3 months"));
	will give "2006-05-12".

truncate() - With overloads for each type, truncates a type to a given
	unit. For example:
	truncate(toDate("2006-08-12"), DatePart.QUARTER);
	will give "2006-06-01", and
	truncate(toDate("2006-03-28"), DatePart.WEEK);
	will give "2006-03-27".

toString() - With overloads for each type, converts to strings
	optionally with a formatting string (Not fully implemented).



A full documentation is available at:
http://peylow.no-ip.org/~peylow/date.html

The source is available at:
http://www.dsource.org/projects/dlisp/browser/branches?rev=51


Comments om implementation, documents and so forth are requested. I have 
no plans to ad any more functions as I like to keep simple things 
simple. But I do plan to add more "DatePart"'s, for example JULIANDAY 
for all the astronomers, and also extract date parts from other 
calendars, such as Arabic etc.

I have intentionaly not included conversion of weekdays and month into 
english names. As I believe that is up to the GUI, and locale parts of 
an applicition, not the low level lib.


regards
	Fredrik Olsson