While I was still in prayer, Gabriel, the man I had seen in the earlier vision, came to me in swift flight about the time of the evening sacrifice.  He instructed me and said to me, "Daniel, I have now come to give you insight and understanding.  As soon as you began to pray, an answer was given, which I have come to tell you because you are highly esteemed. Therefore, consider the message and understand the vision:  

Seventy sevens are decreed for your people and your holy city to finish transgression, to put an end to sin, to atone for wickedness, to bring in everlasting righteousness, to seal up vision and prophecy and to anoint the Most Holy.  

Know and understand this:  From the issuing of the decree to restore and rebuild Jerusalem until the Messiah comes, there will be seven sevens and sixty-two sevens.  It will be rebuilt with streets and a trench but in times of trouble.  After the sixty-two sevens, the Messiah will be cut off but not for himself.  The people of the ruler to come will destroy the city and the sanctuary."  Daniel 9:21-26 written 580 b.c.   

This could very well be the greatest prophecy in the entire Bible.  A decent amount of thought must be expended in order to understand the incredible significance.  What follows is my interpretation.

 This prophecy says three things will occur:

1.  A decree will be issued to restore and rebuild Jerusalem.  
2.  The Messiah will come and be killed.  
3.  A foreign ruler will come and destroy Jerusalem and the temple.

Here is the explanation of the sevens:  

seven sets of seven  7 x 7 = 49
sixty-two sets of seven  62 x 7 = 434

49 + 434 = 483

Once the decree was given to rebuild Jerusalem, there would be 483 years until Messiah was killed.    

Here is the breakdown of the prophecy:

1.  A decree would be issued to restore and rebuild Jerusalem.  This prophecy was written after King Nebuchadnezzar of Babylon (present day Iraq) destroyed Jerusalem in 586 b.c. Daniel had been taken captive to Babylon.  About 50 years later, King Cyrus of Persia (present day Iran) conquered Babylon in 538 b.c.  After another 70 years, a Persian King came to power named Artaxerxes I.  He reigned from 464 b.c. to 425 b.c.  He issued a decree well over 100 years after this prophecy was written:  

It came to pass in the month of Nisan, in the twentieth year of King Artaxerxes, when wine was brought before him.  I took the wine and gave it to the king.  I had never been sad in his presence before.  Therefore, the king said to me, "Why is your face sad, since you are not sick?  This can be nothing but sadness of heart."  I was very much afraid and said to the king, "May the king live forever!  Why should my face not look sad when the city where my fathers are buried lies in ruins, and its gates have been destroyed by fire?"  The king said to me, "What is it you want?"  I prayed to the God of heaven and I said to the king, "If it pleases the king and if your servant has found favor in your sight, send me to the city in Judah where my fathers are buried so that I may rebuild it."  It pleased the king to send me, so I set a time.  I went to Jerusalem.  Nehemiah 2:1-6, 11

The Encyclopedia says King Artaxerxes' reign began in 464 b.c.  This decree took place in the twentieth year of his reign which would be 444 b.c.  

The 483 year period began in 444 b.c.     

2.  The Messiah will come and be killed.   Not only does it tell us the Messiah will come but also that he will be killed.  Many Jews do not feel Jesus is the Messiah because he was killed.  We are told plainly he will be cut off.  Note Isaiah 53:8 where it says the Lord's suffering servant would be cut off from the land of the living.  

If 483 years are added to 444 b.c. (-444), the number comes to 39 a.d.  The Messiah will be killed in 39 a.d. 

However, there is one small problem.  Jewish calendars have 360 days in a year (12 months of 30 days).  Here is one example in Scripture:

They will trample on Jerusalem for 42 months.  I will give power to my two witnesses and they will prophesy for 1,260 days, clothed in sackcloth.  Revelation 11:1-3  (42 months x 30 days/month is 1,260 days) 

Today, we use a Gregorian calendar to tell time.  Gregorian calendars have 365 days in a year.  

483 Jewish years need to be converted Gregorian years:     

483 Jewish years x 360 days/year is 173,880 days.  
173,880 days divided by 365 days is 476 Gregorian years.

483 Jewish years = 476 Gregorian years  

Again, the decree was given in 444 b.c.  476 years from 444 b.c. (-444) is 33 a.d. (there was no year zero)

Using the Gregorian calendar, the Messiah was to be cut off in 33 a.d.

3.  A foreign ruler would come and destroy Jerusalem and the temple.  In 70 a.d., the people of the ruler to come, the Romans under emperor Titus, destroyed Jersualem and the temple.  The Jews have not had a temple since this date almost 2,000 years ago.  

The Messiah had to have come before 70 a.d.    

This prophecy is amazing because it accurately predicts the time the Messiah will be killed more than 500 years before it happened.  It placed his arrival and death between two future events which have both been solidly documented in history.  There is not a total agreement on some of the dates involved in this prophecy.  However, what is not debatable is Jerusalem will be rebuilt, the Messiah will come, the Messiah will be killed and then Jerusalem along with the temple will be destroyed.