That’s the origin of getMonth() in Java (and therefore in JS) returning a value from 0-11 and not 1-12 as many coders initially expect.
What was the origin for this peculiarity in C? That I don’t know, but I’m curious to find out if anyone knows.
Which why the fields of the 'tm' structure [1] (used to represent dates and times) are zero based§. Makes it easy to index into eg an array of day names using tm_wday. I guess at one time Java.util.Date was a leaky abstraction layer on-top of such an api.
§ Except for the tm_mday element, which curiously is 1-based. I've always assumed that this is because it is unlikely to be used as an array index. A long time ago I'm ashamed to admit that I used tm_mday == 0 as a quick check for an invalid tm value.