I would think that "a year" means "a year". I haven't seen any exceptions sited with respect to when Windows 8.1 or Windows 7 SP1 was installed.
I wouldn't expect all the Walmarts, Costcos and Sam's Clubs of the world to empty their shelves of all Windows Computers on August 1st, and....
If you had some type of catastrophic hard drive or motherboard failure of your Windows system, on or about July 29th and was unable to address the problem until sometime in August, I wouldn't expect you to be excluded from the free Update.
I don't think there are any "gotchas" associated with this offer. I truly believe Microsoft wants as many people as possible to be running Windows 10, but.....
That's just me, and that's just my best guess.