Well, without being provided any screenshots of the math involved, I went ahead and did some digging and found the following post.
I don’t know if it’s still the same setup, but assuming it is, my best guess is that time is factoring in somewhere on one of the dates.
Rather than dealing with the 1/1/1970 unix date, I would just TRUNC(Date) the start and end dates. The result is the google style of date numbers, but essentially it’s the same concept, simplifies the setup, and rules out any issues with underlying times.