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According to http://www.clas.ufl.edu/users/ufhatch/pages/03-Sci-Rev/SCI-REV-Home/Historical-Research/Calendars/gregorian_calendar_history.html "The Swedish dithered. In 1700 they began what was intended to be a gradual switch to the Gregorian calendar. They planned to stop observing leap years until their calendar was in line with the Gregorian one. They did omit the leap year in 1700, but observed the leap year in 1704 and 1708 (apparently they forgot the plan). Thus they were 10 days out of step with the Gregorian calendar and 1 day out from the Julian. Then, in 1712, they changed their minds, and went back to the Julian system by adding two leap days to February."

If they had stuck to that plan, when would they have had their next February 29? In 1736? In 1740? Or in 1744?

Robert Soupe
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Their next February 29 would have been in 1744. 1740 would have been too soon.

I stepped through it one-by-one in 4-year blocks, just to be sure:

March 1, 1700 Julian was March 12 Gregorian. But for the Swedes it was March 2, because they had skipped February 29, 1700.

March 1, 1704 Julian was March 12 Gregorian. But for the Swedes it would have been March 3, if they had skipped not just February 29, 1700 but also February 29, 1704.

March 1, 1708 Julian was March 12 Gregorian. But for the Swedes it would have been March 4, if they had skipped not just February 29, 1700 but also February 29, 1704 and February 29, 1708.

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March 1, 1740 Julian was March 12 Gregorian. But for the Swedes it would have been March 12, if they had skipped not just February 29, 1700 but also February 29, 1704; February 29, 1708; February 29, 1712; 1716; 1720; 1724; 1728; 1732; 1736; 1740.

By the time February 28, 1744 had come up on their calendar, they couldn't just have picked up the phone to call Poland, but I'm sure they would have had some way to get in touch and check that their calendars had in fact synced up and they could now have a February 29 the next day.

James47
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