Comment 5 for bug 1201306

The algorithm for determining leap-years after year 14 is not defined.

webtrees currently uses the rule "leap-year every 4 years".

Many alternative algorithms were proposed, but none is "official".
See http://en.wikipedia.org/wiki/French_republican_calendar

If we were to choose the "von Mädler" 128-year algorithm, it would appear to co-incide with the gregorian calendar from 1920-2047.