[Python-Dev] datetime module enhancements

[Tony Nelson]

At 5:45 PM +1300 3/11/07, Greg Ewing wrote:
>Jon Ribbens wrote:
>
>> What do you feel "next Tuesday plus 12 hours" means? ;-)
>
>I would say it's meaningless. My feeling is that subtracting
>two dates should give an integer number of days, and that is
>all you should be allowed to add to a date.

Apple's old MacOS had a very flexible LongDateRecord and date utilities.
Nearly anything one could do to a date had a useful meaning.  Perhaps
Python should be different, but I've found Apple's date calculations and
date parsing to be very useful, in a Pythonic sort of way.

>From old New Inside Macintosh, _Operating System Utilities_, Ch. 4 "Date,
Time, and Measurement Utilities":

Calculating Dates
-----------------
In the date-time record and long date-time record, any value in the month,
day, hour, minute, or second field that exceeds the maximum value allowed
for that field, will cause a wraparound to a future date and time when you
modify the date-time format.

*   In the month field, values greater than 12 cause a wraparound
    to a future year and month.
*   In the day field, values greater than the number of days in a
    given month cause a wraparound to a future month and day.
*   In the hour field, values greater than 23 cause a wraparound to
    a future day and hour.
*   In the minute field, values greater than 59 cause a wraparound
    to a future hour and minute.
*   In the seconds field, values greater than 59 cause a wraparound
    to a future minute and seconds.

You can use these wraparound facts to calculate and retrieve information
about a specific date. For example, you can use a date-time record and the
DateToSeconds and SecondsToDate procedures to calculate the 300th day of
1994. Set the month field of the date-time record to 1 and the year field
to 1994. To find the 300th day of 1994, set the day field of the date-time
record to 300. Initialize the rest of the fields in the record to values
that do not exceed the maximum value allowed for that field. (Refer to the
description of the date-time record on page 4-23 for a complete list of
possible values). To force a wrap-around, first convert the date and time
(in this example, January 1, 1994) to the number of seconds elapsed since
midnight, January 1, 1904 (by calling the DateToSeconds procedure). Once
you have converted the date and time to a number of seconds, you convert
the number of seconds back to a date and time (by calling the SecondsToDate
procedure). The fields in the date-time record now contain the values that
represent the 300th day of 1994. Listing 4-6 shows an application-defined
procedure that calculates the 300th day of the Gregorian calendar year
using a date-time record.

Listing 4-6     Calculating the 300th day of the year

PROCEDURE MyCalculate300Day;
VAR
    myDateTimeRec:      DateTimeRec;
    mySeconds:          LongInt;
BEGIN
    WITH myDateTimeRec DO
    BEGIN
        year := 1994;
        month := 1;
        day := 300;
        hour := 0;
        minute := 0;
        second := 0;
        dayOfWeek := 1;
    END;
    DateToSeconds (myDateTimeRec, mySeconds);
    SecondsToDate (mySeconds, myDateTimeRec);
END;

The DateToSeconds procedure converts the date and time to the number of
seconds elapsed since midnight, January 1, 1904, and the SecondsToDate
procedure converts the number of seconds back to a date and time. After the
conversions, the values in the year, month, day, and dayOfWeek fields of
the myDateTimeRec record represent the year, month, day of the month, and
day of the week for the 300th day of 1994. If the values in the hour,
minute, and second fields do not exceed the maximum value allowed for each
field, the values remain the same after the conversions (in this example,
the time is exactly 12:00 A.M.).

Similarly, you can use a long date-time record and the LongDateToSeconds
and LongSecondsToDate procedures to compute the day of the week
corresponding to a given date. Listing 4-7 shows an application-defined
procedure that computes and retrieves the name of the day for July 4, 1776.
Note that because the year is prior to 1904, it is necessary to use a long
date-time record.
-- 
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