The filing date marks the USPTO’s receipt of a patent application wherein the application includes:
- a specification description and, if the application is non-provisional, at least one claim and,
- any required drawings (if parts are missing the USPTO can refuse to assign a filing date).
If the patent application is filed by express mail with a proper certificate of mailing, the filing date is when the Post Office accepted the application, otherwise it’s the date the application was received in the USPTO mail room.