From the client's perspective, patent attorneys may appear to use the expressions "filing date" and "priority date" interchangeably. These phrases actually have very different meanings.
A filing date is simply the date a patent application was filed in the Patent Office.
A priority date (sometimes called an effective filing date) however, is the earliest claimable filing date for an application.
Consider these two examples:
In that scenario, the regular application has a 2013 filing date, but a January 2012 priority date.
The second inventor's application has Jan. 1, 2013 as both a filing date and priority date .
Having an early priority date can be important.
If a prior art reference dated June 15, 2012 is dicsovered, anticipating the inventions, the first inventor could rely on his January 1, 2012 priority date to get around the prior art. The second inventor could not.