Dating 1 year
Pope Gregory XIII addressed this by essentially skipping 10 days in the date, making the day after October 4, 1582, October 15.
M41 as the Lord Commander of the Imperium, he launched the Indomitus Crusade to push back the encroaching Forces of Chaos that had been unleashed across the galaxy after the birth of the Great Rift.
As the Indomitus Crusade drew to a close around 111.
M42, Guilliman calculated the current year by the five main factional variants of the Imperial Calendar to be anywhere between the early 41st Millennium and an entire millennium later, and that was leaving out the numerous lesser, more heretical interpretations.
The Primarch was determined to make a full and accurate accounting of the Imperium's fragmentary and often contradictory history, and so he commissioned the formation of a new organsation -- the Logos Historica Verita.
It was charged with the daunting task of discovering, collating and cataloging thousands of standard years of human history that had been lost or was woefully inaccurate due to superstition, suppression or purposeful obfuscation by the High Lords of Terra.