I hadn't read the paper, just the article. Which is primarily what I was commenting on. When I read "now researchers have identified cells in the human brain that make this sort of memory possible", I was lead to believe that NPR was suggesting that there weren't plausible mechanisms of timekeeping for episodic memory before now. Especially since they don't mention any other mechanisms, and Buzsáki (not a study author) and NPR together imply that people with hippocampal lesions who can't order events correctly are simply missing their time cells.

Anyhow, I've now read much more of the two linked papers, and some related papers.

In the 2011 paper linked from the NPR article, under "Neuronal Ensembles Signal Time, as well as Location and Behavior, during the Delay Period" (https://www.sciencedirect.com/science/article/pii/S089662731...) "time cells" are defined under their relation to "place cells". As far as I can tell, this paper coined the term. Now, in this paper, they aren't sure if the time cells are scalar time (Gibbon) or non-linear time (Staddon and Higa) or some combination of the two. Well, Gibbon's scalar expectancy theory for timing (SET) uses pacemaker cells in the theory. Now other papers do want to move away from pacemaker cells: Staddon and Higa point out that the "Weber law" that SET depends on doesn't scale well, so they have other theories. However, I could still imagine that pacemaker and timestamping neural circuits would work together, no?

This review article of timing in the brain seems helpful, though I don't have time to read it fully today: https://www.sciencedirect.com/science/article/pii/S089662731... It covers oscillators (pacemaker-accumulator/SET), ramping, and population clocks ("time cells" and other names). I find it problematic that the NPR article only focuses on one name for one possible explanation of the observed behavior. It makes for a nice article, and the metaphor of time cells and place cells are attractive. It doesn't leave

Time cells seem to invoke a chain of neurons firing, to encode events along the chain. I can believe that this orders events on the order of tens of seconds (the literature surveys from milliseconds to tens of seconds). But the NPR article suggests that the ordering of an entire tour of UCSD is also encoded in time cells. I doubt that there was one unending chain of ordering for that entire tour. Personally, I find that I have to reason about time on longer time scales, or I might mix up a recollection of how a story goes. (Once I tell a story a few times, I've learned a new skill--how to tell the story--so it becomes easier to recall a sequence I've chosen to highlight.)