The traditional Roman martyrology gives the date of creation as 5,199BC.  This is not a date that anyone would come up with by using the Vulgate bible.  Hence St Bede, basing himself on the Vulgate, calculated the date as 3,592BC.  The date on the martyrology apparently derives from some version of the Septuagint, from which the Latin version of the bible anterior to the Vulgate derives.  Eusebius of Caesarea placed this date into his Chronicon, which was translated into Latin by St Jerome around AD 378.  See here for a reasonably learned study, which is however strangely lacking a footnote for the reference to Bede.

Ven. Mary of Agreda says that she was told by the Blessed Virgin that 5,199 was the date of creation.  Her superior or spiritual director, I forget which, told her to ask again, and Mary of Agreda says that she was again told plainly that this was the correct date.

There was a wide-spread belief in the early patristic period that the world as we know it would last 6,000 years, and that this would be followed by a thousand year reign of Christ and the saints.  This is inspired, among other things, by Apoc. 20:22 – “And he laid hold on the dragon the old serpent, which is the devil and Satan, and bound him for a thousand years.”  I’ve given some examples here.

One cannot help being impressed by the fact that, starting from the date on the martyrology, six thousand years would bring us to AD 801, and that Charlemagne was crowned by the pope as the first holy Roman emperor on Christmas day 800.  Was not this a reign of Christ on earth?  Likewise, it is impressive that the holy empire was brought to an end a thousand years later by Napoleon who became first consul in 1799 and extinguished it over the next few years.

For the end of the world was long ago,
And all we dwell to-day
As children of some second birth,
Like a strange people left on earth
After a judgment day.