After reading the excellent post by Farkel I thought I may be able to contribute.
This principle [year day] is held as a biblical principle by Adventists, Jehovah's Witnesses and the Worldwide Church of God. It is also rejected by most Adventists scholars also administraters and members.
The nineth chapter of Daniel is used by some to try to prove the existance of the [year day] as it is clamed that it foretells the comming of the Messiah. So is the year day necessary here? Most textbooks of bible hermeneutics do not even mention the possibility of a year day principle.
Ok Daniels Seventy Weeks - Why Days Equal ''Years'' in this unique case before looking into whether Scripure teaches or supports the use of a Year-Day Principle, in this case nearly all Bible scholars agree that days do represent years. The days are presumed days of course as the prophecy doesnt actually use the word days it uses weeks and can also be translated 'sevens'.
So why are the seventy weeks correctly interpreted as seventy weeks of years or 70 times 7 years?
Daniel's audience would be familier with the three following idears.
The first is the seventy -year captivity was a punishment [Jeremiah 25:4-11]
The second is the concept of an original punishment being multiplied by seven[Leviticus 26:14-39]
THe third concept is that of weeks of years [Leviticus]
The fourth idear is that Daniel employed figurative language where it says in Daniel two that the third kindom will rule over the world. This is not true of any historical empire. In fact many scholars feel that the number seven is used symbolically in Daniel 3,4 and 9.
Although the first three idears lead commentators to agree that seventy weeks of years is meant idear number four tells us we should not necessarily look for exactness any more than the furnace in Daniel 3 was heated exactly seven times hotter.
So in this case of seventy weeks days do represent years [presumed days admittedly] and they do not because of a year-day principle but because of the above reasons.
Application of the Year -Day Principle in the Interpretation of Scripture
The principle is applied very unevenly. According to Adventists Daniel 8[2300 days], Daniel 7,12 and Revelation 12 [time and times and half a time] Revelation 11 and 13 [forty two months] Revelation 11 and 12 [1260 days] Daniel 12 [1290 days] and Daniel12[1335 days] In the JW understanding it is appied in Daniel 4 and 9. However there are many places where predictive prophecy is given , and number of days given and it is not applied.
In some cases, when the rule is not applied the predictive prophecies are not in apocalyptic sections. these include;
Jesus predicted he would be arrested in two days; Math12 - Jesus predicted he would be buried for three days Math 27-Jesus predicts his resurrection after 3 days.
Jonah prediction of the overturning of Nineveh in forty days
In three cases where predictive prophecies occur in apocalyptic the year day Adventists are themselves divided.
Rev.2-Smyma is to suffer tribulation 10 days , some apply 10 years here
Rev 11 The two witnesses are applied to the time religion was outlawed during the french revolution others apply it to 1260 years and the JWs apply it to the secretary and treasure of the WT society being jailed from 1914-18 from memory.
Finally it is not applied to the thousand years of Revelation 20 if it was it would be 360,000 years.
Two pragmatic Rules
[1] When a predictive prophecy includes a stated number of days, and application of the year day principle would appear plausible to a significant percentage of people , the principle is applied.
[2] When a predictive prophecy includes a stated number of days , but application of the principle would be viewed as silly, the principle is not applied. eg the buriel arrest and resurrection.
Those who beleive in the principle often fail to apply it consistently and uniformly. They sometimes apply it to times not stated in days and do not apply it in cases where days is stated in a predictive prophecy.
Barry