Month

Month = 30 Days

Over the last 5,000 years, the shortest and longest durations of the lunar month were 29.27 and 29.84 days, and the mean duration was 29.53 days.

Because of Biblical inclusive counting, the Bible counts the duration of the month as 30 days, since the partial day after the 29th day is counted as a full day.

Genesis

The Bible shows 1 month to be 30 days by equating 5 months with 150 days.

5 x 30 days = 150 days.

Gen. 7:11 In the 600th year of Noah’s life, in the 2nd month, on the 17th day of the month, on the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.

Gen. 8:2-4 The fountains also of the deep and the windows of heaven were stopped, and the rain from heaven was restrained; and the waters returned from off the earth continually: and after the end of 150 days the waters decreased. And the ark rested in the 7th month, on the 17th day of the month, upon the mountains of Ararat.

Revelation

The Bible shows 1 month to be 30 days by equating 42 months with 1,260 days.

42 x 30 days = 1,260 days.

Rev. 11:2-3 And the court which is without the Temple leave without, and measure it not; for it hath been given unto the nations: and the holy city shall they tread under foot 42 months. And I will give unto my two witnesses, and they shall prophesy 1,260 days, clothed in sackcloth.

Philo

Philo says the month always has 30 days due to inclusive counting.

For days when the new moon is sighted after sunset at the very beginning of the 30th day of month, since there is a short duration of time after sunset before the sighting of the new moon, that short duration of time is counted as an entire day, the 30th day.

And because of inclusive counting, the day of the sighting of the new moon shortly after sunset is also counted as an entire day, the 1st day.

Therefore, the 30th day of the month and the 1st day of the month overlap on the same day when the new moon is sighted after sunset at the very beginning of the 30th day.

“And the expression ‘from,’ has a double sense. One, that by which the starting point from which it begins is included; the other that by which it is excluded. For when we say that from morning to evening there are 12 hours, or from the new moon to the end of the month there are 30 days, we are including in our enumeration both the first hour and the day of the new moon. And when any one says that such and such a field is 3 or 4 furlongs distant from the city, he clearly means to leave the city itself out of that measurement.”
——Philo, On Dreams, Book 2, Section 257, Written c. 30 AD.

Extra Day

In order to keep the 1st day of the month synchronized with the new moon, the Bible calendar adds a full 30th day to the month if the new moon is not visible at the very beginning of the 30th day of the month.

Since a Biblical day starts after sunset and since the Biblical month can only have 29 or 30 “full” days:
1) If the new moon is sighted after sunset at the very beginning of Day 30 of Month 1, then that day is counted as Day 30 of Month 1 as well as Day 1 of Month 2;
2) If the new moon is not sighted after sunset at the very beginning of Day 30 of Month 1, then that day is counted as Day 30 of Month 1 and the following day is counted as Day 1 of Month 2.

The Jews called the months that had the extra day an “intercalated month.”

“R. Judah says, ‘As to a New Year at which one suspected [the month] might be intercalated [so that the New Year would be observed on the 30th of Elul and on the 1st of Tishre], a man prepares two erubs and says, My erub for the first day is at the east, and for the second is at the west.'”
——Mishnah, Eruvin, Chapter 3, Verse 7, Written c. 200 AD.

“One who slaughters a heifer and divides it [among purchasers] on the New Year, if the month was intercalated [if the last month of the Sabbatical year was given an extra day], [the money owed to the butcher] is cancelled [by the Sabbatical year]. But if [the month was] not [intercalated], [the money owed to the slaughterer of the heifer] is not cancelled [because the slaughtering did not take place during the Sabbatical year].”
——Mishnah, Shevi’it, Chapter 10, Verse 2, Written c. 200 AD.

“[If] the court and all the Israelites saw [the new moon], the witnesses having been examined, but they had no chance to say, ‘It is sanctified,’ before it [actually] got dark, lo, this [month coming to an end] is an intercalated month.”
——Mishnah, Rosh Hashanah, Chapter 3, Verse 1, Written c. 200 AD

“[The decision to] intercalate the month is before three [judges].”
——Mishnah, Sanhedrin, Chapter 1, Verse 2, Written c. 200 AD.

“[If] one [of the witnesses] says, ‘It was on the 2nd of the month,’ and one of the witnesses says, ‘It was on the 3rd of the month,’ their testimony stands, for one of them may know about the intercalation of the month, and the other one may not know about the intercalation of the month. [If] one of them says, ‘On the 3rd,’ and one of them says, ‘On the 5th,’ their testimony is null.”
——Mishnah, Sanhedrin, Chapter 5, Verse 3, Written c. 200 AD.

“[If] he ate in an association formed for a religious duty, [if] he ate on the occasion of the intercalation of the month, [if] he ate food in the status of second tithe in Jerusalem.”
——Mishnah, Sanhedrin, Chapter 8, Verse 2, Written c. 200 AD.