Date command: A command-based date computation engine
datec allows you to use "date commands" to modify datetime's by adding to them, like datetime.datetime.now() + Period(2, 'week').
A date command can be parsed from string using the parse() function, which create a command from a string representation. This forms the basis of the datec command, which is a command-line program to output datetime after applying date commands. In general the date representation is NxYYYY-mm-ddTHH:MM:SS.ffffff, where unspecified parts are omitted leaving the symbols intact, like "2x-2-29T3::." (see the following for the meaning). If fractional part is not specified the "." may be omitted, if all time parts are not specified the "T::." can be omitted, if all date parts are not specified the "--T" can be omitted, and if N is 0 the x may be omitted. By there are a couple other more formats like +3week and -2wed for shifting by period and weekday.
Date commands are in two forms: period shifting commands and partial datetime shifting commands. The first type is more familiar: they look like
- +2week (shift the datetime forward by 2 week)
- -1month (shift the datetime backward by 1 month)
Period is one of year, month, week, day, hour, minute and second, represented by an object of the Period class. Fractional numbers are acceptable except for year and month. If shifting a period leads to an invalid date (e.g., shift backward 1 month from 2019-07-31), it moves backwards the closest valid date (here, 2019-06-30). In general the parts finer than the shifted part is unaffected (e.g., shifting 1 month from 2019-07-31 02:00 gives you 2019-06-30 02:00).
Partial datetime shifting is less familiar. It looks like:
- 12:: (set the hour number to 12)
- +2x12:: (move forward to the second hour 12)
- +4x--31 (move forward to the fourth occurrence of day 31 of a month)
- -3x-02-29 (move backward to the third occurrence of February 29)
- wed (set to the Wednesday of the same week, week starts on Sunday)
- -3wed (move to the third Wednesday before the current datetime)
They are represented by either a Weekday object or a PartialDate object with a count. A count of 0 means setting instead of shifting. Only integer counts are acceptable.
It is an error to set to an invalid date (e.g., --31 applied on 2019-06-25 is an error). The datetime parts which are specified must be consecutive (it is an error to specify 12::05). It is also an error to shift for occurrence of partial date with year specified (e.g., "+2x2019--").
On the other hand, shifting to an invalid date with day number specified will shift more until a specified date is valid. For example, if you add -2-29 with count 1 to 2019-01-01, you end up with 2020-02-29, because 2019-02-29 is not a valid date. If the count is 2 you get 2024-02-29 instead.
Shifting to invalid date by month will cause the date to moved backwards until the date is valid. E.g., if you shift by -6- with count 1 (next June) from 2019-05-31, you get 2019-06-30. With count 2 you get 2020-06-30.
This library is grown out of frustration that it is tedious to have a shell script or program to get a datetime like "the next 6pm from now" or "the next 3rd of any month from two days ago". With this module they can be specified like "+1x18:00:00.0" and "-2day +1x--3" respectively. In the expected use cases, counts are small numbers. So the library is not always efficient (at times we just loop "count" times to step forward or backward). Whenever it is simple to do so, the implementation just forward to relativedelta, in which case they are more efficient.
At present the program does not handle timezone and daylight saving. This is bacause the author lives at a place where no daylight saving is observed. Contributions are welcome.
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