Why do some Christians interpret Daniel's 1260 day prophecy as days and not years?
Whether a day is interpreted as a literal day or as a year depends on the context. Interpreters looked at the 70 sevens of the Daniel 9 and almost unanimously decided it cannot be literal days, but it could be years. (However, some interpreters duck the whole issue and go for an indeterminate length of time.)
With the 1290 days (Daniel 12:11) and 1335 days (Dan 12:12), though, the context suggests actual days. How could an individual patiently wait for the coming of 1335 years? (Dan 12:12)
Daniel 8:14 says 2300 "evening-mornings": some think that means 2300 days, and some, such as William Hendriksen, think it means a total of 2300 sacrifices, half in the morning and half in the evening making 1150 days.
Hendriksen then says this is the exact number of days that the sacrifices stopped in the days of Antiochus Epiphanes, though where he gets that information I do not know.
On the best evidence, the Temple was desecrated on the Seleucid Calendar date of 15 Kislev 145 (1 Maccabees 1:54) which is 6th Dec 167 BC, Julian date; and was cleansed on the Seleucid Calendar date of 25th Kislev 148 (1 Maccabees 4:52) which is 14th Dec 164 BC, Julian date.
This actually amounts to 1104 days (not 1150 days).
The conversion of Seleucid Calendar dates to Julian dates is taken from "Babylonian Chronology - 626 BC to AD 75" by Richard A. Parker and Waldo Dubberstein. See also "From Abraham to Paul - A Biblical Chronology" by Andrew Steinmann, page 217.
A time, times, and half a time
"A time, times, and half a time" (Dan 7:25, 12:7, Rev 12:14) is introduced in the book of Daniel. It is seen again in the book of Revelation in figures relating to three and a half years, such as "forty two months" (Rev 11:2, 13:5) and "one thousand two hundred and sixty days" (Rev 11:3, 12:6). These are all symbolic references to the same time period, where, it is true, each month is spoken of as having thirty days.
What does it mean? The best explanation I know is summarised here:
The three-and-a-half symbol has a structure of one, two, and a half. What the "one time" represents (a period of tribulation), the "two times" also represents except that there is a doubling, intensifying or prolonging. Likewise the "half a time" represents the same thing again, but there is a limiting or cutting short. https://www.simplybible.com/f919-revln-three-and-a-half-symbol.htm
All that needs to be added is that it is our Lord God, our loving heavenly Father, who will cut short these periods of tribulation. These periods are not outside of God's control, but he is allowing them to fulfil his purposes. The blood of the martyrs is the seed of the church, and the sufferings of this present time are not worthy to be compared with the glory that will be revealed in us (Romans 8:18).
Sometimes "a time, times, and half a time" is converted to days. So in Daniel 9:27 there is a week made up of two periods of three and a half days, and in Revelation 11:9-11 we have two references to a period of three and a half days.
In all cases, whether for years or days, "three and a half" is a symbolic period which refers to a period of suffering or persecution. It may also refer to a period of chastening of God's people for particular sins.
[In the days of Elijah the people suffered (an actual, non-symbolic) three and a half years of famine (Luke 4:25, James 5:17).]
In all of these cases trying to turn these periods from symbolic years or days to literal years or days is inadmissible. If I divide 1260 apples for 360 people I will never get a certain number of oranges. It is misleading to call these "prophetic years", it is probably better to call them "symbolic years", or "symbolic days": the length of time cannot be calculated, all we know is that they will be times of trouble, persecution and suffering.
It is noteworthy that 1290 days is one month longer than the symbolic three and a half year period of 1260 days, and 1335 days is 75 days or two and a half months longer. I think it follows that these numbers are symbolic days only... it is inadmissible to try to turn these periods - 1260, 1290 and 1335 days - into actual lengths of time.
It also suggests that the week in Daniel 9:27 consisting of two periods of three and a half days is a symbolic week.. it is inadmissible to try to turn this week into a specific length of time. Rather, it is identifying a period of intense persecution, double trouble, for the people of God. This would fit neatly with the view that the final 70th week in Daniel 9:25-27 is the entire Gospel Age, starting with the Word of the Lord coming to John the Baptist (Luke 3:1-3) and ending with the Second Coming of our Lord. [In Daniel 9:24 the 70th week is part of an actual length of years, but not in Daniel 9:25-27.]
The coming of the Kingdom of God, far from being a period of great dominion of God's people, in its first phase up to the Second Coming of our Lord, will be a period of persecution, sometimes intense, for God's people.
Blessed is he that waits and comes to the thousand three hundred and five and thirty days (Daniel 12:12).