This notion still exists today, months of 30 days in the Hebrew Calendar are called "full" and those with 29 are deemed to be "deficient." In addition to their declaring the beginning of each month based upon a sighting of the new moon, priest-astronomers were also charged with pinpointing the start of a year.
Instead, they returned to a fixed-length calendar that had 12 months of 30 days each, but with five days added at the end.
10 Usually at a date later than the mid-eighth century B. E., many other peoples who had previously considered the year to be 360 days in length reluctantly returned to a calendar of twelve 30-day months, but added five days to the end of their year. with the birth of their founder, the prophet Zoroaster, use a calendar of 365 days.
This resulted in a change to their method of approximating year length that had been in use for nearly a millennium.
But it also caused them to wonder where the additional days came from.
It contained twelve months of 30 days each and an extra five days at the year’s end.