P6's crazy calendar

I date vs duration complications I cannot figure out.  I assume it is from the contractors's calendar.  I am going to import others to see.  But also, will all time be calculated correctly if I put the hours per time period at 8dy 40 wk 172 mo, 2000 a year.  It is odd p6 makes you do this because each month is a different amoun 30/31 dys month etc. Is there a source as to a good basic setting for 5 dy wk , 6dy wk, 7dy wk.