Daniel 9:25 Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: the street shall be built again, and the wall, even in troublous times.

There are four references in the Old Testament to the rebuilding in Jerusalem:

If you will closely examine the four passages, you'll find that none of the first three decrees contain any explicit reference to the rebuilding of the city itself, and offer no specific date of the decree. The main direction for the first three decrees is for the rebuilding of the Jewish Temple.  Only Nehemiah 2 specifically mentions rebuilding the city, and only Nehemiah provides a date--the month of Nisan in the 20th year of Artaxerxes' reign.  Since Artaxerxes Longimanus reigned 465-425 BC, this decree would have been issued in 445 BC [The Coming Prince, Robert Anderson].  Traditionally, the lack of a day notation would indicate the first day of the stated month, or in this case, 1 Nisan 445 BC.

The Hebrew word shabuwa` appears in the Old Testament 20 times and is translated "week" 19 times, and "seven" one time [Greek-English Concordance To the New Testament, J. B. Smith].  According to Strong's Concordance, it means "seven, period of seven (days or years), heptad, week" Since a 24-hour day would make absolutely no sense in this context, it is generally understood by everyone except amillennialists to mean weeks of years.  Compare this to Genesis 29:27 where shabuwa is understood to equal seven years:

 Genesis 29:27 Fulfill her week <07620>, and we will give thee this also for the service which thou shalt serve with me yet seven other years.

But the next question becomes, how many days in a year?  In order to calculate the length of this prophecy, one must know both the number of years, and the length of each year of those years.  Fortunately, the length of a prophetic year can easily be established from both the Old and New Testaments: