There are a few interesting twists with regards to patent application filing dates. For example, an applicant may apply for a new patent and try to claim the filing date of a patent they had previously filed. Remember, the earlier the filing date, the more likely an inventor is to get his idea patented before any competitors do. Also, the earlier filed application could in fact prevent the new application from being granted since two patents can never be granted for the exact same invention.

Therefore, the applicant will have to claim priority of the earlier filed application in order to receive the second patent at all.

An inventor may claim the priority benefit of a previously filed foreign application or one that was previously filed in the U.S (these are called foreign and domestic applications, respectively).

There are two different laws covering these alternatives. 35 U.S.C. 119 subsection (e) and 35 U.S.C. 120 discuss claiming priority of domestic applications. 35 U.S.C. 119 subsections (a-d & f) discuss claiming the priority of foreign applications.

Either way you file it, there must be at least one common inventor between the two applications.

Domestic priority claims must occur while the two applications are copending. That means the earlier filed application must not be abandoned or allowed to issue, rather it must be pending.

Foreign priority claims must occur within 12 months from the date of the earliest foreign filing.