datefmt: Define 1 year as 365.2425 days instead of 365.25

datefmt: Define 1 year as 365.2425 days instead of 365.25

According to sqlite3, it's much more accurate. For example:

    SQLite version 3.43.1 2023-09-11 12:01:27

    sqlite> select datetime(julianday('2023-09-26')-6000*365.25);
    -3977-08-12 00:00:00
    sqlite> select datetime(julianday('2023-09-26')-6000*365.2425);
    -3977-09-26 00:00:00

    sqlite> select datetime(julianday('2023-09-26')+7000*365.2425);
    9023-09-26 12:00:00
    sqlite> select datetime(julianday('2023-09-26')+7000*365.25);
    9023-11-18 00:00:00

I chose 365.25 days back in the day because it aligns with the
astronomical year, offers greater consistency with fewer fractions, and
is a widely accepted standard in scientific contexts. However, for
improved accuracy, it's better to use the Gregorian year rather than the
Julian year.

Here's the whole experiment session, compressed with gzip+base64:

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c1fbd928-5c80-11ee-ab26-83850402c3ce