People commonly claim a prophetic year in the Bible has 360 days — 12 months of 30 days. This makes no sense at all. A year is about 365 days. No matter how many days in a month it still takes the same amount of time for the earth to go around the sun. In every calendar ever invented the regular cycle of months are adjusted in some way so the dates line up with the seasons. It takes about 29–1/2 days for the moon to go around the earth, not 30 days.

I think people might have invented this notion of 360 days in a prophetic year to make it easier to do the calculations but it doesn't reflect the actual calendar in use at the time (nor of any calendar in use ever.)

I'm not impressed with the argument that 3–1/2 years of 360 day years = 1,260 days = 42 months of 30 day months. There is another passage mentioning 1,290 days and 1,335 days.

The Babylonian calendar had months matching the length of the lunar month, certainly not 30 days, with added time periods to make the yearly seasons fall in the same months.

The moon rotates about the earth in about 29–1/2 days. Therefore, the Jewish lunar calendar has months of 29 days and 30 days averaging out to 29–1/2 days. A month is not 30 days (although some months are 30 days.) For example, the months of Iyar, Tammuz, Elul, Chesjvan, Marcheshvan, and Adar have 29 days. Israel had various feasts based on the phase of the moon so a strict 30 day month would not work.

From Genesis: The flood ended exactly 5 monthsafter it began,lasting a total of 150 days.Yes, 5 x 30 = 150. But note that the ark rested on the mountain in one verse but the flood waters abated in the other so these two end points are not necessarily the same. In addition, three of these intervening months only have 29 days so the five months totals to 147 days, not 150 days.