The leading zero and 0x notation is only used for integers, so the presence of a decimal point changes the game so to speak.
While the months are not part of the language, they are part of the time.h suite of the standard library - which is defined byt he standard. K&R are very careful to point out that the months number represents the "months since January" (italics in original). Harbison and Steele point out the same distinction - which probably came from unix.