I actually doubt the extinction were as total or global, on no less than three fronts. First up is palynology shows global disruption to food webs at the K/Pg, though it was not equally severe everywhere. Floras of the Antarctic Kingdom bounced back soon enough. Second is the matter needs 'slices of time' recorded in the sedimentary rocks, that must be suitably fossil yielding, from at least the late Masstrichtian to early Danian - Hell Creek is very informative for this reason, but cannot represent a global picture. Third is the size filter selecting K/Pg survival, really ought to have let through a number of Campanian-Maastrichtian coelurosaurs. The smaller alvarezsaurs were apt to survive, as were petite long tailed pennaraptorans such as Hesperonychus and Rahonavis. Not a lot is known of Palaeocene birds still. It's still possible that toothed Ichthyornis type forms, and even enantiornitheans survived. It's obvious the latter included more than only avisaurids, in the latest Cretaceous. No one same would deny dinosaurs were victims of the K/Pg but the way people think about that, involves a fair few assumptions that can't be justified simply by repetition. More on reddit.com

Dinosaurs weren’t necessarily hit hardest (though larger creatures definitely were hit hard). Even among mammals it was brutal. Whole groups were wiped out (North American metatherians, for example). Pterosaurs were wiped out. Only half of the crocodyliforms made it. Belemnoids and ammonoids got wiped out. Mosasaurs and plesiosaurs were gone. People focus on the dinosaurs, but pretty much anything big got hammered, as did most of the specialists. The dinosaurs that made it (a small fraction of the birds) were small generalists able to eat seeds and bugs. That’s how extinctions go. Niches get emptied and then refilled. More on reddit.com

The simple answer is the rocks they are found in. The earth is in layers, the deeper you go the more layers there are, and within these layers are smaller layers. If you pay attention to them and study them you can start to be able to piece Earth's history together, and with tools like carbon dating, we can guesstimate when and where they would've died off. Some are more obvious than others, but in the end I hope I was able to answer your question mate More on reddit.com

It looks like the dinosaurs were in decline for several million years before the impact event at Chixulub. As best we can tell the extinction of the large therapod and sauropod dinosaurs happened instantaneously, geologically speaking. That might mean days, months or even decades or more in reality; rock preservation in most places does not work at human timescale resolutions. Ir could be as long as centuries or millennia. I would personally put my money on decade to century scale; with only pockets surviving the first year, and perhaps some isolated pockets lasting over 1000 years (similar to mammoth post ice age). Edit: turns out my dinosaur knowledge was a few years out of date. Strike out the first sentence. Edit2: Following the OP edit, u/stringoflights adds the following and asked me to add it here for reference: Birds didn’t just evolve from dinosaurs, they are dinosaurs, and dinosaurs did not go extinct. We have more dinosaur species alive today than mammal species. It’s fine to pose a question about non-avian dinosaurs, but the fact that birds are dinosaurs is important when we are studying patterns of extinction. The bird “evolving back” from extinction that is mentioned in the edit is an example of iterative evolution. It is absolutely not a reason to ignore an entire radiation of dinosaurs, and it’s pretty important that that misconception is corrected. Iterative evolution means that the same or similar structures arose from the same source population at different times. It doesn’t mean the wholesale evolution of the same species twice. In the case of the Aldabra rail, which is what the edit mentions, the white-throated rail has colonized the island from nearby and evolved flightlessness more than once. More on reddit.com