think the 1260 comes from 42 months. then 42 months is close to 3.5 years...

"The time period for these beliefs is also based on other passages: in the Book of Daniel, "time, times, and half a time", interpreted as "three and a half years," and the Book of Revelation, "a thousand two hundred and threescore days" and "forty and two months" (the prophetic month averaging 30 days, hence 1260/30 = 42 months or 3.5 years). The 1290 days of Daniel 12:11, (rather than the 1260 days of Revelation 11:3), is thought to be the result of either a simple intercalary leap month adjustment, or due to further calculations related to the prophecy, or due to an intermediate stage of time that is to prepare the world for the beginning of the millennial reign.[4]"

I think the 1260 comes from 42 months. then 42 months is close to 3.5 years...

"The time period for these beliefs is also based on other passages: in the Book of Daniel, "time, times, and half a time", interpreted as "three and a half years," and the Book of Revelation, "a thousand two hundred and threescore days" and "forty and two months" (the prophetic month averaging 30 days, hence 1260/30 = 42 months or 3.5 years). The 1290 days of Daniel 12:11, (rather than the 1260 days of Revelation 11:3), is thought to be the result of either a simple intercalary leap month adjustment, or due to further calculations related to the prophecy, or due to an intermediate stage of time that is to prepare the world for the beginning of the millennial reign.[4]"