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Genealogy: Using the Right Calendar

Find tips on using the correct calendar when figuring dates.

Genealogy: Using the Right Calendar

When the glossy new calendars start arriving in December, it probably doesn't occur to you that New Year's Day was not always 1 January. Furthermore, it may not be obvious how this can affect your genealogical research.

Calendars were developed to make sense of the natural cycle of time: days and years from the solar cycle, months from the lunar cycle. It took some experimentation before folks got it to the current system. There are many calendars, but for right now, we need be concerned only with the Julian and Gregorian calendars.

The Julian calendar resulted from Julius Caesar's reformation of the system to conform more closely to the seasons. The Gregorian calendar was Pope Gregory XIII's solution for the gradual problem that had developed with the Julian calendar: over time the calendar was 10 days off the natural solar cycle. To compensate, the Gregorian calendar dropped 10 days from October in 1582. And to keep this problem of extra days from reoccurring, one day was added to February in every year divisible by 4.

Genie Jargon

Double dating is the practice of giving dates, from 1 January, through February, to 25 March before 1752, two dates to represent the old and the new calendar, for example 23 January 1749/50. This may alternately be shown as 23 January 1749 O.S. (old style) or 23 January 1750 N.S. (new style).

The Gregorian calendar was adopted by different nations at different times. It was generally adopted by Britain and her colonies in 1752. The day added to the calendar every four years (leap year) meant that the calendar was now 11 days out of sync with the solar cycle. To take care of this, the system was adjusted so that the leap day is dropped from every century mark not divisible by 4. Instead of dropping 10 days in October, the British dropped 11 days in September and changed the New Year from 25 March to 1 January. Why do you need to be aware of this interesting bit of trivia? The calendar change makes dates in the months of January, February, and up to 25 March, prior to 1752, subject to double dating.

George Washington's birthday, 11 February 1731, under the old calendar became 22 February 1732 under the new calendar, and would be expressed as 22 February 1731/32. This doesn't actually change the date of his birth, merely the way it is expressed. In 1731, February was almost the end of the year because 1732 began on 25 March. After the year 1752 there are two things to contend with: the dropping of 11 days, and the change of the beginning of the year from 25 March to 1 January.

The calendar change affects your research because it is sometimes hard to determine whether the dates are meant to be old style or new style. You may think that the change was not significant enough to make a difference in your research, but it does. If you find records that indicate Abraham was born on 27 March 1741 and his younger sister Ruth was born on 23 March 1741, you may think there is something wrong. In reality, it is likely correct, because 23 March of 1741 in the old style calendar followed 27 March 1741 by about 12 months.

Tree Tips

Though the difference of 11 days can explain some records (he died on the third of the month but was not buried until the thirteenth), genealogists should not convert dates to account for the 11-day difference unless the old style date would cause confusion. If it does, change it, but indicate that the date has been conformed to the new style calendar. Or leave it as is and explain the seeming discrepancy.

The change in calendar can also explain the seemingly erroneous court item that shows the will was dated 3 December 1740, and proved in court 1 January 1740. “That can't be,” you think. But it can, and was, because 1 January 1740 followed 3 December 1740 (expressed as 1 January 1740/41). Once you understand this, you need to show it in your records, or others will think you have erred. The best way to show it is this: the will was dated 3 December 1740, proved in court 1 January 1740/41. Be sure to use a slash and not a dash; it is the slash that clarifies you are referring to the double dates caused by calendar changes.

Don't just convert the date with no explanation. If you do prefer to express it in new style (N.S.), show it as “1 January 1741 N.S.” so that others understand under which calendar the date is given. For more on calendar history see

It is important to interpret carefully when faced with dates that were shown in months. This prevalent Quaker custom was also used by some others. A date of “30 9ber 1741” (or “30 9 mo 1741”) is 30 November 1741, based on the calendar of the time in which the first month was March and the ninth month was November. It is best, in extracting records that are expressed in months, to write them in your abstract as shown in the original record. If you want to show it also as it would be under the present calendar, add that in brackets. For example, John Betts was born, according to the Quaker Monthly Meeting record, 30 9ber 1741 [30 November 1741 N.S.].

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