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[[Category:Developers/General]][[Category:Documentation]]
This page documents advanced features of GRAMPS dates.
From [2]: "For example, England and the American colonies didn't officially accept the new calendar until 1752. Before 1752, the English government still observed March 25 as the first of the year, but most of the population observed January 1 as the first of the year. For this reason, many people wrote dates falling between January 1 and March 25 with both years..."
Sometimes, a dual date may appear as a fraction, as in this grave stone(170 and 3/4, which means 1703 and 1704):
[[Image:Gravestone.jpg]]
* 1721/2
* 1719/20
* 1799/800
These slash-years can appear anywhere in a date that a regular year can appear.
* Jan 20, 1750 (Julian,Mar1)
* Feb 23, 1710/1 (Mar25)
Note that if new year's day is not Jan 1, then January may come after December that year!
Dates with new year day codes will be sorted appropriately. For example, using the Python Gramplet:
<pre>
> dates = ["Dec 31, 1720", "Jan 1, 1720", "Mar 1, 1720", "Mar 25, 1720", "Mar 26, 1720", "Dec 1, 1720"]
> dates.sort(cmpkey=lambda a,bx: cmp(Date(a).sortval, Date(b).sortvalx))
> dates
['Jan 1, 1720', 'Mar 1, 1720', 'Mar 25, 1720', 'Mar 26, 1720', 'Dec 1, 1720', 'Dec 31, 1720']
> dates.sort(cmpkey=lambda a,bx: cmp(Date(a x + " (Mar25)").sortval, Date(b + " (Mar25)").sortval))
> dates
['Mar 25, 1720', 'Mar 26, 1720', 'Dec 1, 1720', 'Jan 1, 1720', 'Dec 31, 1720', 'Mar 1, 1720']
</pre>
This creates a list of date strings. You can sort them by wrapping Date() around them, and using their sortvalcomparing to another Date. In the first example, they have a normal, January through December ordering.
In the second example we append a " (Mar25)" after the date. Now when we sort, we see that the year begins with Mar, followed by all of the months up to Dec, then Jan, Feb and the first part of March is last.
Likewise, date differences can also be made with this method. For example, consider the two dates, '''Jan 1, 1750''' and '''Dec 1, 1750'''. Without considering what was the first day of the year with normal sequencing, those dates are 11 months apart. However, if they are actually separated by the new year, then they are actually only a month apart, and the first follows the second:
<pre>
> Date("Dec 1, 1750") - Date("Jan 1, 1750")
11 months
> Date("Dec 1, 1750 (Mar25)") - Date("Jan 1, 1750 (Mar25)")
-1 month, -1 day
</pre>
== Date Math ==
The Date object in GRAMPS 3.1 has been enhanced in the way that dates are compared to one another. There are actually three different uses for comparing dates:
# for sorting (eg, for listing in a table)
# for describing (eg, for describing an age)
# for ranking (eg, for computing "record holding" spans)
=== Date Differences ===
When a date is subtracted from another, a Span is created. A date Span can be used for creating a textual representation of the difference. A Span can also be used for ranking the difference, including a minimum difference, and a maximum difference.
=== Date Comparisons ===
There are three methods for seeing if one date happened before another date:
# simple method: date1.sortval < date2.sortval
# more sophisticated and expensive: date1.match(date2' "<")
# more restrictive: date1.match(date2, "<<")
Likewise for seeing if two dates are the same:
# simple method: date1.sortval == date2.sortval
# more sophisticated and expensive: date1.match(date2' "=")
and for seeing if one date happened after another date:
# simple method: date1.sortval > date2.sortval
# more sophisticated and expensive: date1.match(date2' ">")
# more restrictive: date1.match(date2, ">>")
= References =
# http://www.genealogy.com/00000771.html
# http://en.wikipedia.org/wiki/New_Year
