Old Style / New Style Date Converter

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Age CalculatorCousin CalculatorJulian / GregorianOS/NS ConverterRoman NumeralsPerpetual CalendarCensus Age
OS/NS (Old Style/New Style) refers to the change in when the new year began. Before adoption of the Gregorian calendar, the legal new year in many countries started on March 25 (Lady Day) rather than January 1. This means dates recorded between January 1 and March 24 carried the previous year's number in contemporary records — this calculator adjusts for that difference.

This issue is most relevant to English and Welsh records, where both the new year change and Gregorian adoption happened simultaneously in 1752. Most other countries resolved the January 1 new year much earlier. The adoption dates below indicate when each country switched to the Gregorian calendar and January 1 as new year:
Most of Catholic Europe: 15 October 1582 (papal decree)
Scotland: 1 January 1600 (new year only; Gregorian in 1752)
England & Wales: 14 September 1752
Russia: 1918
Greece: 1923 (last European country)
Common examples
1588MDLXXXVIIISpanish Armada
1776MDCCLXXVIUS Independence
1888MDCCCLXXXVIIILongest Roman numeral ≤1999
1945MCMXLVEnd of WWII
1752MDCCLIIGregorian adoption (England)
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