More correctly the formatting function uses the Lilian Date. The difference between a Lilian Date and a Julian Date is that the "Julian" epoch occurs at noon at the prime meridian, but a "Lilian" has an epoch which occurs at noon at some other (local) meridian. The internal datetime structure does not carry the meridian offset of the Lilian Date it stores relative to the prime meridian, so calculation of the "time zone offset" is not possible. Also, the functions do not use UTC. They use "Universal Time" time in which a minute always contains exactly 60 seconds, and hour contains exactly sixty minutes, a day contains exactly 24 hours, and a year contains exactly 365 days (except when it contains exactly 366 days). The length of a "second" is variable in order to maintain these invariant. UTC allows a minute to contain 60 seconds "give or take a few" in order to approximate (the C in UTC stands for "on average approximately") UT1 time using fixed length seconds.