C uses zero-based array indexing. A C array variable is a pointer to its first element, and an element index is just an offset from that. Element 0 is at start

C uses zero-based array indexing. A C array variable is a pointer to its first element, and an element index is just an offset from that. Element 0 is at start + 0, element 1 is at start + (1 * element size) etc. 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. [1] https://en.cppreference.com/w/c/chrono/tm https://en.cppreference.com/w/c/chrono/tm