A year is based on the revolution of the Earth around the Sun, a month is originally based on the motion of the Moon around the Earth, a day is based on the rotation of the Earth on itself. There is no beginning and no end, like the seasons, the end of a cycle is the beginning of a new one.
The Mayans had two calendars: Haab (365 days) and Tzolkin (260 days). Tzolkin is cyclical instead of linear.
The first roman calendar was composed of 310 days spreaded throughout 10 months.
Two more months were added a bit later, Januarius, Februarius for a total of 354 days.
In order to be more astronomically accurate as well as for military reasons, the year ended up being 365 days long for three years in a row with a fourth year being 366 days long.
The Celts also had a more circular view of time than linear. Each day started at Sunset and finished at next day’s sunset.
Iraq: The Babylonians (in today’s Iraq) had a calendar composed of 12 lunar months. Each of these months had either 29 or 30 days. Which means a years would be 354 days.
All the modifications to the Roman calendar caused the year 46 BC to be 445 days long in order to restore the spring equinox to its traditional date of March 25. A real mess!