The only contemporary sources for Bishop Nechtan are charters; he appears as "Nectan escob Abberdeon" in a Gaelic charter recorded in the notitiae on the Book of Deer, a charter which explicitly dates to "the eighth year of the reign of David", that is, 1131.
[1] He also appears in a charter granted to him by King David I of Scotland, a charter which the modern editor dates to 1137.
[2] Later Medieval authorities make specific claims about dates.
Hector Boece, for instance, wrote that Nechtan became bishop in 1122, and that Nechtan ruled Mortlach for 14 years, and a further 17 years at Aberdeen.
Moreover, Gavin Dunbar, a 16th-century Archbishop of Glasgow, wrote in his Epistolare that Nechtan's see was moved from Mortlach to Aberdeen in the year 1125, partially contradicting the account of Boece.