40 days are an appetizer with respect to the records accomplished by Maurizio Montalbini:
https://en.m.wikipedia.org/wiki/Maurizio_Montalbini
Montalbini spent a total of two years and eight months underground since he started his experiments in the 1980s
Interesting, though the article itself says nothing about any findings, other than that time seems slower in an environment with less stimulation, which is probably obvious.
There was a similar experiment done some time back. IIRC, the subjects eventually settled on to a 27-hour daily cycle, rather than our normal 24-hour cycle.
That seems to be occurring here too. With them needing a longer objective time to match a 'standard' subjective time.
It seems that the opposite is true (they underestimated the time spent in the cave): 40 days seemed like 30 days for most and even 23 days for one volunteer, according to The Guardian article (https://www.theguardian.com/world/2021/apr/25/deep-time-team...).