Over time, countries have switched calendars. You may find two dates listed during the time when the calendar was changing.
For example, while the European countries were switching from the Julian calendar to the Gregorian calendar, from 1582 to 1752, the dates from January through March may have been recorded with a dual year, and sometimes a dual day or month.
See the project instructions and field helps on how to handle dual dates. If there are no guidelines, list both dates, separating them with the word “or,” such as 1843 or 1848.
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