There are times in biblical prophecy when a day symbolizes a year. The clearest example of this principle is the ‘seventy sevens’ prophecy in Daniel 9, where instead of a literal 70 weeks or 490 days, the prophecy is universally interpreted as lasting 490 years.
Other examples in Scripture include Numbers 14:34, where the Israelites were sentenced to wander in the wilderness for 40 years, “one year for each of the forty days you explored the land.” Also Ezekiel 4:4-5, where the prophet is told to lie on his left side and put the sin of Israel on himself “the same number of days as the years of their sin. So for 390 days you will bear the sin of the people of Israel.”
Historicists apply this biblical principle to the “Time, Times and Half a time,” “3 1/2 years,” “1260 days”, and “42 month” time periods, which occurs seven times in Daniel and Revelation. Therefore, instead of being a literal 1260 days, they are interpreted as symbolizing 1260 years.