New patents (resulting from patent applications filed after 1995) are enforceable from the date they are granted by the US Patent Office PTO until 20 years from the date they were filed.  Old  patents were enforceable for 17 years from the date they were granted.  So in short,  new-based on filing date;  old - based on grant date.  Sometimes patent applications can languish in the PTO system based on PTO inaction, the need for extended analysis of prior art references, and/or stall tactics by applicants.  Prior to 1995 these stall tactics did not punish the applicant because patents were enforceable for 17 years from the grant date of the patent.  In contrast, such stall tactics are not rewarded because new patents run from the application date and not the grant date; thus any stalling results in less patent term.  So, old, but not granted, patent applications are often referred to as submarine patent applications because they could surface at any time and torpedo your business strategy and patenting plans due to their secrecy until granting.  Because these submarines could lurk for a long time, new technologies could be introduced and gain acceptance before these submarines could surface - and devastate unsuspecting ships . . . or industries . . . .   Due to the passage of time since 1995 there are very few of these submarines left to surface but according to Dennis Crouch in the linked article, 450 of these patent applications have somehow lasted to this day, kind of like a dinosaur still hanging around.  20 of these monsters issued in 2013 (owned by 7 different entities).  Here is hoping no submarines surface in your backyard.