Clocks roll back at 2am Pacific time just like everywhere else - so the author's explanation seems unsatisfying although I've encountered this type of bug before.
[1] https://www.nist.gov/pml/time-and-frequency-division/popular...
[2] At present, daylight saving time in the United States begins at 2:00 a.m. on the second Sunday of March (at 2 a.m. the local time time skips ahead to 3 a.m. so there is one less hour in that day) [and] ends at 2:00 a.m. on the first Sunday of November (at 2 a.m. the local time becomes 1 a.m. and that hour is repeated, so there is an extra hour in that day.)
2 am local time is the transition time. Some days you go from 1:59 am standard to 3:00 am daylight, others you go from 1:59 am daylight to 1:00 am standard.
It's not like leap seconds which are added (or hypothetically removed) at a specific universal time, so whatever your time zone, whatever minute corresponds with utc 23:59 gets an extra second.
The Temporal spec[1] should alleviate a lot of the JS induced pain. Knowing how to correctly store and manipulate time data, however is a completely separate issue that even really experienced engineers can easily mess up, even with solid STD lib support.