This is the calendar that applies during the two periods of universal judgement in the Bible, the Flood and the tribulation.

Two passages in the Bible tell us that these periods have 30-day months.

The flood

Genesis 7:11,24; 8:3-4 says that from 17/2 to 17/7 was 150 days...

"In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, on that day all the fountains of the great deep were broken up, and the windows of heaven were opened... And the waters prevailed on the earth one hundred and fifty days... the end of the hundred and fifty days the waters decreased. Then the ark rested in the seventh month, the seventeenth day of the month..."

The tribulation

Revelation 12:6,14; 13:5 tells us that 1260 days are 3½ "times" and 42 months. These months are each 1260/42 = 30 days long, and the "times" are 1260/3.5 = 360 days long.

"Then the woman (Israel) fled into the wilderness, where she has a place prepared by God, that they should feed her there one thousand two hundred and sixty days...But the woman was given two wings of a great eagle, that she might fly into the wilderness to her place, where she is nourished for a time and times and half a time... And he (the beast) was given a mouth speaking great things and blasphemies, and he was given authority to continue for forty-two months..."

360-day "Prophetic Year"

A so-called "prophetic year" of 360 days is often held to be the Bible's calendar year. However, it is based on twelve 30-day months which are only mentioned twice in the Bible - during the 150 days of the flood deluge and during the second half of the tribulation. Revelation 12:14 calls these 42 months of 30 days "3½ times". The 360-day year is a Babylonian "time", and is never referred to in scripture as a year.

It is interesting that both the 150 days of the flood deluge and the 42 months of the second half of the tribulation, when "times" are used for dating in the Bible, are periods of judgement!