Old Style and New Style dates

Old Style and New Style dates refer to a dating system adjustment that occurred in various countries over several centuries. This change was necessitated by the switch from the Julian calendar to the more accurate Gregorian calendar. The Julian calendar, introduced by Julius Caesar in 46 BC, had a year length of 365.25 days, which did not accurately reflect the actual solar year of approximately 365.2425 days. This discrepancy caused the calendar to drift with respect to the equinoxes, leading to the incorrect timing of Easter and other calendar-based religious observances.

The Gregorian calendar, proposed by Aloysius Lilius and instituted by Pope Gregory XIII in 1582, corrected this drift by introducing a new method of leap year calculation and omitting several days to realign the calendar with the seasons. Countries adopting the Gregorian calendar skipped a number of days to correct the accumulated drift from the Julian calendar: for example, in 1582, the day after October 4 was October 15 in countries that adopted the Gregorian calendar immediately.

The terms "Old Style" (O.S.) and "New Style" (N.S.) indicate the calendar system used in dating events. "Old Style" refers to the Julian calendar, and "New Style" refers to the Gregorian calendar. The adoption of the Gregorian calendar occurred at different times in different countries, leading to dates being recorded differently according to the calendar in use at the time and place. This has implications for historical research, genealogy, and the interpretation of historical documents.

For instance, in the United Kingdom, the change from the Julian to the Gregorian calendar occurred in 1752, with the day following September 2, 1752, becoming September 14, 1752. This change also aligned the start of the year to January 1; previously, the year in England and its colonies began on March 25, a practice known as Lady Day being the New Year.

The difference between the two calendar systems has decreased over time due to the additional day that the Julian calendar accumulates every 128 years. As of the 21st century, the difference stands at 13 days.

Understanding the distinction between Old Style and New Style dates is crucial for accurately interpreting historical documents and events. It is especially important in genealogy and historical research, where precise dating can significantly impact the interpretation of events and relationships.

See also

• Calendar (New Style) Act 1750
• Leap year
• Timekeeping

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