system
Given the following post, respond as if you're commenting on Reddit:
human
[40K] Does Imperium know history before Age of Strife, like world wars etc. and if not, how did calendar survive?

^ Is Imperial calendar even same as ours by the way? Is "40k" really 40k?
gpt
The Imperial Calender is actually pretty complex; it would have to be to take into account all the different time zones and planets it encompasses, as well as any timey-wimey shenanigans involving the Warp. All time is still kept in "AD" like our calendar and is based on the date as taken from Earth, so time is still familiar, but it uses a system of "checks" to determine the relative accuracy of the date. 

For example, a given date might be 01121015 M.41, meaning it's November the 21st, the year 015 of the 41st Millennium. The 0 at the beginning means this date was taken on Earth, and thus is perfectly accurate. A 1 would mean it was taken within the Sol System and is thus still reasonably accurate, and the numbers rise from there. 2 is direct psychic contact with Sol, 3 is indirect contact with a 2, and so on and so forth of degrees of separation from Earth timekeeping.

The Adeptus Administratum use an alternate calender which seperates years into 1000 part equal parts, rather than using months and days. It's mostly just for record keeping, most citizens of the Imperium use the more traditional one.