This point that the Russian calendar ran some days behind the western version argued by Chandler and others however may be another myth.
According to Robert Goetz
French agents are known to have supplied misinformation to Mack, which might explain Mack's failure to discover what would appear to be obviously faulty figures by simply looking at the distances on a map. While some authors have suggested that the 12-day difference between the Julian Calendar used by Russia and the Western Gregorian Calendar provides an explanation, there is no evidence that this was the case and no record of any difficulties in coordinating calendars in this campaign or in the 1799 campaign.
1805: Austerlitz: Napoleon and the Destruction of the Third Coalition,Greenhill Books, 2005, p.50.