[roger_smith]

I guess I should have learn2google....

Of course the MyBB docs talk about it.

Averages
The time (referred to below as time) used to calculate these averages is the difference between the current time and when the first user (usually UID 1, the administrator who installed the forum) was created, in days.

Posts per day
This is calculated by dividing the total number of posts by the time (see above).