舊式與新式日期

舊曆（英語：Old Style、縮寫 O.S.）與新曆（英語：New Style、縮寫 N.S.）分別代表西方國家更換曆法前後的兩套不同日期系統，以表示1582年至20世紀期間各國從儒略曆過渡到格里曆時出現的日期差異。

英格蘭、威爾斯、愛爾蘭和英屬北美於1752年曾變曆兩次；首次將新一年的開始時間從淑女節（英語：Lady Day）（3月25日）調整為1月1日（蘇格蘭從1600年開始執行），而第二次則於9月的日曆中刪除11天，以徹底廢儒略曆並改行格里曆^[1]^[2]。為了適應此兩次變曆，作家們使用雙重日期（英語：Dual dating），根據兩種日期風格來確定某一天的日期。

對於俄羅斯一類無調整日期變曆的國家，O.S.和N.S.只是表示儒略曆和格里曆的曆法系統，許多東正教國家的教會出於宗教目的，甚至持續沿用儒略曆至今。

參考文獻

^ Poole 1995，第95–139頁.
^ Spathaky, Mike Old Style and New Style Dates and the change to the Gregorian Calendar （頁面存檔備份，存於網際網路檔案館）. "Before 1752, parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the Historical Year 1734 started even though the Civil Year 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Civil or the Historical Year. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."

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[FOOTNOTEPoole199595–139-1] Poole 1995，第95–139頁.

[MS2-2] Spathaky, Mike Old Style and New Style Dates and the change to the Gregorian Calendar （頁面存檔備份，存於網際網路檔案館）. "Before 1752, parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the Historical Year 1734 started even though the Civil Year 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Civil or the Historical Year. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."

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