The Anniversary stats are broken in that whoever’s service record you go to, it will always show your own stats. Whenever I go to someone else’s service record, it shows my stats.
As for the times, I think they are calculated based on date and time started and finished, not time actually spent in the mission. For example, if you are playing a mission and then quit, if you come back a month later and complete the same mission, it will say that it took you 30 days from the time that you started the mission to the time that you finished it.
There are a couple missions that say Waypoint says took me 566 days to finish.