Archaeologists have pinpointed the construction of Stonehenge to 2300BC - a key step to discovering how and why the mysterious edifice was built.
The radiocarbon date is said to be the most accurate yet and means the ring's original bluestones were put up 300 years later than previously thought.
The dating is the major finding from an excavation inside the henge by Profs Tim Darvill and Geoff Wainwright.
The duo found evidence suggesting Stonehenge was a centre of healing.
Others have argued that the monument was a shrine to worship ancestors, or a calendar to mark the solstices.
A documentary following the progress of the recent dig has been recorded by the BBC Timewatch series. It will be broadcast on Saturday 27 September.
Date demand
For centuries, archaeologists have marvelled at the construction of Stonehenge, which lies on Salisbury Plain, Wiltshire.
Mineral analysis indicates that the original circle of bluestones was transported to the plain from a site 240km (150 miles) away, in the Preseli hills, South Wales.
This extraordinary feat suggests the stones were thought to harbour great powers.
Trench
The dig was the first inside the ring since 1946
Professors Darvill and Wainwright believe that Stonehenge was a centre of healing - a "Neolithic Lourdes", to which the sick and injured travelled from far and wide, to be healed by the powers of the bluestones.
They note that "an abnormal number" of the corpses found in tombs nearby Stonehenge display signs of serious physical injury and disease.
And analysis of teeth recovered from graves show that "around half" of the corpses were from people who were "not native to the Stonehenge area".
"Stonehenge would attract not only people who were unwell, but people who were capable of [healing] them," said Professor Darvill, of Bournemouth University.
"Therefore, in a sense, Stonehenge becomes 'the A & E' of southern England."
Modern techniques
But without a reliable carbon date for the construction of Stonehenge, it has been difficult to establish this, or any other, theory.
Until now, the consensus view for the date of the first stone circle was anywhere between 2600BC and 2400BC.
To cement the date once and for all, Professors Darvill and Wainwright were granted permission by English Heritage to excavate a patch of earth just 2.5m x 3.5m, in between the two circles of giant sarsen stones.
In the stone socket
The key was to get organic matter from the bluestone sockets
The dig unearthed about 100 pieces of organic material from the original bluestone sockets, now buried under the monument. Of these, 14 were selected to be sent for modern carbon dating, at Oxford University.
The result - 2300BC - is the most reliable date yet for the erection of the first bluestones.
Strictly speaking, the result was rounded down to "between 2400BC and 2200BC" - but 2300BC is taken as the average.
An even more precise date will be produced in the coming months.
"It's an incredible feeling, a dream come true," said Professor Wainwright, formerly chief archaeologist at English Heritage.
"We told the world we were going to date Stonehenge. That was a risk, but I was always confident," said Professor Darvill.