What is the difference in priority claim requirements for applications filed before and after September 16, 2012?

The requirements for filing a foreign priority claim differ for applications filed before and after September 16, 2012. The key difference lies in where the priority claim must be presented:

• For applications filed on or after September 16, 2012: The priority claim must be presented in an Application Data Sheet (ADS). This is specified in 37 CFR 1.55(d)(1): For applications filed under 35 U.S.C. 111(a) on or after September 16, 2012, 37 CFR 1.55(d)(1) requires the claim for priority to be presented in an application data sheet.
• For applications filed before September 16, 2012: The priority claim can be presented either in an Application Data Sheet or in the oath or declaration. As stated in the MPEP: For applications filed under 35 U.S.C. 111(a) prior to September 16, 2012, unless provided in an application data sheet, the oath or declaration under 37 CFR 1.63 must identify the foreign application for patent or inventor’s certificate for which priority is claimed under 37 CFR 1.55, and any foreign applications having a filing date before that of the application on which priority is claimed, by specifying the application number, country, day, month, and year of its filing.

Regardless of the filing date, the priority claim must include the same essential information: the application number, country (or intellectual property authority), and filing date (day, month, and year) of the foreign application. The difference is primarily in the document where this information must be presented.