[Python-Dev] datetime module enhancements (original) (raw)

Tony Nelson tonynelson at georgeanelson.com
Sun Mar 11 17:02:39 CET 2007

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.

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.

TonyN.:' <mailto:tonynelson at georgeanelson.com> ' <http://www.georgeanelson.com/>

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