Saint Drostan (also sometimes spelled as Drustan, Dustan or Throstan) lived, very approximately, from 560 to 630. He was a follower of Saint Columba who was active in Aberdeenshire. The wider picture in Scotland at the time is set out in our Historical Timeline.
Saint Drostan is only know about at all because he appears in two old manuscripts, the Book of Deer (or Book of Deir) an illuminated manuscript whose origins date back to the 900s, and Breviarium Aberdonense, (the Aberdeen Breviary) produced by Bishop William Elphinstone in Aberdeen in 1510, though the two do not tell a completely consistent story about him. His name is also attached to various churches and to other features, such as St Drostan's Well at Aberlour in Speyside, now the water source for Aberlour Distillery.
St Drostan seems to have been a son of the royal family of Dalriada, whose father was called Cosgrach. Presumably being a younger son, he was trained to become a monk on Iona by St Columba. He accompanied Columba on at least one of his missions to the Picts, and together they travelled to Aberdeenshire, possibly with St Fergus.The Pictish King they were visiting gave Columba a site for a monastery at Deer. Having worked together to set up the monastery, Columba appointed Drostan to be its abbot, and returned to Iona. Drostan later became Abbot of "Dalquhongale" Abbey, which some have associated with Holywood Abbey in Dumfries and Galloway. Still later he became a hermit in Glen Esk in Angus, and is said to have performed many miracles, including restoring the sight of a priest named Symon. After his death, Drostan's relics were kept in a church at Aberdour, close to the site of the village of New Aberdour in northern Aberdeenshire.
The dates for all this revolve around two fixed points. The first is the suggestion in the documentary sources that Drostan was abbot of the monastery at Deer in 600, and the other is the date of the death of Columba, which happened in 597. It therefore seems reasonable to suggest a foundation date for the monastery at Deer in the 580s. The original monastery at Deer fell out of use and when Deer Abbey was founded by William Comyn, the Earl of Buchan, in 1219 on a site half a mile to the west. Drostan's original monastery was probably on the site later used for Old Deer Old Kirk.