Dogmaus: for a short time.
Hence why I do use the program I mentioned with the ones I do bother to get ... that gives you an installer you can use any time.
But like I also said, it seems about half of the games they provide, don't require same day activation, despite what they say on the GGOTD page for the game. I'm not entirely sure, but when the downloaded zip file doesn't contain a Setup.gcd file the game doesn't require activation ... at least with the few I've checked.
Dogmaus: So if you uninstall it's gone and you must rebuy it from itch.io if you want to play it again, or on a new computer. The general quality of those games and the annoying activation system are something that has made me lose every interest.
From what I have seen, the Jurij games don't come with the encrypted Setup.gcd file, so can be installed any time ... though I will confirm that, because I can't recall if I tested them in particular.
EDIT I can confirm that last game,
Shooter Arena In Ruins, was just a setup.exe and readme.txt file, and installed fine without needing activation, though when I tried to run it, it complained about my version of Windows ... this is 32 bit Win 7 and not my gaming PC. I also wasn't connected to the web when I did it, and no complaint about that. So not a complete test, but not the usual GGOTD online activation confirmation before install allowed. But I guess when the game starts it might want to check online. The readme.txt file also only has one sentence, unlike the multiple sentences of the other ones, that mention all the activation stuff and time limit.