Excel Help--Finding the Average Duration within the past month

nini8519 There appear to be a number of issues with that formula including wrong () groupings and trying to AND your AVERAGEIF with other conditions AND trying to subtract 2 COUNTIF statements. Try this (I didn't actually test it but should be close if not right):

=AVERAGEIFS('Onboarding Interface Tracker'!T6:T285,
            'Onboarding Interface Tracker'!T6:T285, ">0",
            'Onboarding Interface Tracker'!S6:S285,">"&EOMONTH(TODAY(),-2),
            'Onboarding Interface Tracker'!S6:S285,"<="&EOMONTH(TODAY(),-1)
            )

  is that >0 criteria required?

As for past year you can use similar format and use EOMONTH(..., -13) for the past 12 months or use that DATE(...) format you were using.  (Using DATE() wasn't a problem except that you had misaligned (), I just switched to EOMONTH because it is cleaner and easier to read)

Hope that helps