Patio,
One point that should be made concerning Daniel's "seventy weeks" prophecy is that it doesn’t actually use the word "days." It speaks of "weeks," or more precisely "sevens." The Hebrew word which is translated as "weeks" in many Bibles is translated as "sevens" in others. Why? Because that Hebrew word actually refers to a "group of seven," not just a group of seven days, as in the word "week," but a group of seven anything. So Daniel 9 actually refers to "seventy groups of seven" years and makes no reference to "days" at all. This being so, the so-called "seventy weeks" prophecy cannnot be used to support the "year-day principle."
One article discussing (debunking) the so-called "year-day principle," with more reasons why Daniel 9 cannot be used to support it, can be read at this web address:
http://www.goodnewsunlimited.org.au/yr_day.htm