If clouds obscured vision on the thirtieth day, a new month was declared to have begun.
When month lengths were identical with lunations, only those that lasted 30 days were considered to be normal.
In Persia under the Sassanids, and in Armenia and Cappadocia the official system of time-reckoning was twelve months of 30 days followed by five more days at the end of the year.
However, Arabian astronomers said the Sassanian year of twelve 30-day months was adjusted to the seasons by intercalating a month every 120 years.
The 360 day calendars had been in use for the greater part of a millennium.
E., civilizations all over the world either discarded or modified their old 360 day calendars.
Two eastern Mediterranean peoples who did not embrace Islam were early Christians in upper Egypt, whom we now call Copts, and their neighbors to the south, the Ethiopians. Their years are divided into 12 months of 30 days each, and the extra five or six days are added after the twelfth month. Each of the thirty days as well as each of the gatha days has its own name.Their calendar, composed of alternate 29-day and 30-day months, kept roughly in step with the lunar year.To balance the calendar with the solar year, the early Babylonians calculated that they needed to add an extra month three times every eight years.This system required the addition of an extra month three times every eight years, and as a further adjustment the king would periodically order the insertion of an additional extra month into the calendar.The Babylonians, who lived in what is now Iraq, added an extra month to their years at irregular intervals.