When Was Stonehenge Built?

It is thought that Stonehenge was started around 3100 years BC but that the building of it actually took place in three phases, over a period of 1600 years.   
The first phase involved a circle 115 feet across formed by a low bank surrounding a ditch, then another bank inside the ditch.  Then in around 2100 BC a double circle of 80 large bluestones were erected.  These bluestones are reputed to have been dragged from south west Wales, which is about 130 miles away.
In the third phase which was between 2000 and 1550 BC, the bluestones were removed and a ring of 30 upright sandstones were erected.  These were linked with horizontal lintels at the top.  Later however, the bluestones were re-erected.
No one actually knows for certain what the purpose of Stonehenge was.  Whilst it  does seem to have an association with Druids, the work was finished some 1000 years before the Druids actually came to Britain.

A simple question but unfortunately not a simple answer !

The earthwork and foundations were started around 3100BC, and the original edifice was of timber. This was replaced by stone around 2600 BC and the configuration we see today dates from around 2200BC.