I have written that the 360 day year was quite common in the ancient world of Daniel’s day and that is true enough, but the strongest reason to use the 360 day year is because of how the Bible specifies this time length. To understand this though we need to see the passage of Daniel’s ‘sevens’ in its entirety – given here
24 ‘Seventy “sevens” are decreed for your people and your holy city to finish transgression, to put an end to sin, to atone for wickedness, to bring in everlasting righteousness, to seal up vision and prophecy and to anoint the Most Holy Place.
25 ‘Know and understand this: from the time the word goes out to restore and rebuild Jerusalem until the Anointed One, the ruler, comes, there will be seven “sevens”, and sixty-two “sevens”. It will be rebuilt with streets and a trench, but in times of trouble. 26 After the sixty-two “sevens”, the Anointed One will be put to death and will have nothing. The people of the ruler who will come will destroy the city and the sanctuary. The end will come like a flood: war will continue until the end, and desolations have been decreed. 27 He will confirm a covenant with many for one “seven”. In the middle of the “seven” he will put an end to sacrifice and offering. And at the temple he will set up an abomination that causes desolation, until the end that is decreed is poured out on him. (Daniel 9:24-27)
When we do this we see that the entire passage speaks of 70 sevens, not just 69 (which we only need to deal with in calculating the arrival of the Christ). This passage speaks of a final ‘seven’ which comes after the ‘desolations have been decreed’. This desolation was the destruction of Jerusalem and exile of the Jews from it in 70 AD. For 1900 years Jerusalem was desolate. So this 70th seven was not to follow consecutively from the end of the 69th week. It would follow much later, and is still in our future. The passage dealing with the 70th seven speaks of a ‘he’ who will break a covenant in the ‘middle of the seven’. This ‘he’ is obviously an antagonist since he will break a covenant and put an end to sacrifice and offering.
There is a parallel passage in Daniel 7 that fits with this passage. It reads
The fourth beast is a fourth kingdom that will appear on earth. It will be different from all the other kingdoms and will devour the whole earth, trampling it down and crushing it. 24 The ten horns are ten kings who will come from this kingdom. After them another king will arise, different from the earlier ones; he will subdue three kings. 25 He will speak against the Most High and oppress his holy people and try to change the set times and the laws. The holy people will be delivered into his hands for a time, times and half a time. (Daniel 7:23-25)
This also is looking to the future when a ‘king’ will oppress the ‘holy people’ (ie Jews) by trying to change the ‘set times and the laws’. This is fits very well with the previous passage where ‘he’ will ‘break a covenant’. Then it says in Daniel 7 that the Jews ‘will be delivered into his hands for a time, times and half a time’. If this is dealing with the same event as above where the covenant is broken in the ‘middle of the seven’ the ‘time, times and half a time’ would be the 3.5 years of the half sevens (“time, times and half a time” = 1 + 2 + 0.5 = 3.5 = “middle of seven” = 3.5)
These things have not yet taken place and Revelation, the last book of the Bible, looking to this same coming event, adds to these passages using the image of Israel as a woman and Jesus as her offspring, by stating that
She gave birth to a son, a male child, who ‘will rule all the nations with an iron sceptre.’ And her child was snatched up to God and to his throne. 6 The woman fled into the wilderness to a place prepared for her by God, where she might be taken care of for 1,260 days….
7 Then war broke out in heaven. Michael and his angels fought against the dragon, … 9 The great dragon was hurled down – that ancient snake called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, …
13 When the dragon saw that he had been hurled to the earth, he pursued the woman [Israel] who had given birth to the male child [Jesus]. 14 The woman was given the two wings of a great eagle, so that she might fly to the place prepared for her in the wilderness, where she would be taken care of for a time, times and half a time, out of the snake’s reach. (Rev 12:5-14)
John, the writer of Revelation, describes the same time of distress for the Jewish people that Daniel had described and uses the same phrase ‘time, times and half a time’ but also specifies that this is 1260 days.
In the previous chapter John also states that
But exclude the outer court; do not measure it, because it has been given to the Gentiles. They will trample on the holy city for 42 months. 3 And I will appoint my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth. (Rev 11:2-3)
Here 42 months = 1260 days (= 30 day months = 360 day years). And this describes the same period when the Gentiles have broken the covenant and trample on the city.
All these passages in Daniel and Revelation seem to be describing the same event. In that case the ‘half-seven’ would be 1260 days and therefore the year length used for the 70th week of Daniel is 360 days. And if the 70th Seven of Daniel is composed of 360 day years it only makes sense that the other ‘sevens’ are also composed of 360 day years. This is Biblical context by which we use 360 day years in calculating the coming of the ‘Christ’