When the Christian Religion was introduced into Russia in 988 AD it brought with it the Byzantine Christian calendar, which operates according to the Julian system but numbers its years according to a supposed date for the creation of the world in what we would now describe as 5508 BC (or BCE). The Latin names for the months of the year supplanted the vernacular Slavic names and New Year's Day was assigned to 1 March.
In the mid-14th century the Russian ruler Simeon the Proud was persuaded by the Metropolitan Theognostos that a more proper date for the creation of the world would be 1 September 5508 BC, and 1 September duly became the official New Year 's Day, although apparently this was not universally adopted. …
Ellis, Charles. "Russian Calendar". The Literary Encyclopedia. First published 25 September 2008
[https://www.litencyc.com/php/stopics.php?rec=true&UID=5547, accessed 19 March 2019.]