In reference to the patent: US634042
The filing date of a patent is the date the documents get filed at the respective patent office. So it's exactly what one would think. The patent term is normally 20 years from the date of filing.
A source for confusion is introduced when talking about the priority date.
It is possible to claim priority to an earlier application. That's what's done with provisionals, divisionals, PCT applications ...
This essentially means that one files a second patent after a first patent. The first patent has a first filing date, while the second patent has a second filing date. But the second patent, if it claims priority to the first patent can get the priority date that is the filing date of the first patent. In this case, the priority date is used to asess novelty, inventive step and first-to-file-priority. Still the filing date of the second patent triggers the 20 year patent term, effectively allowing 21 years of protection.
Then there are divisionals and continuations. Those don't exactly claim priority of an earlier patent, they claim the earlier patents filing date, which effectively changes the patent term to less than 20 years from the date of filing the divisional/continuation.
Additionally, to overcome a double patenting objection in the US, one can add a terminal disclaimer, effectively shortening the patent term without changing the filing nor priority date, but having a similar effect.
Usually the filing date and the priority date are the same. There are a few exceptions including:
- Continuation patents
- Domestic patents based on foreign or international filings
- Patent filings based on US provisional applications
This website provides more detail.