I have values for Jan to Dec for Year 1 in Table 1 and values for Jan to Dec
for Year 2 in Table 2.

I want to calculate averages in a 3rd table using source data from Tables 1
and 2.

Table 3 also has columns Jan to Dec for Year 2 but the value for each month
is based on values 6 months after the respective month and 6 months before
the month.

So in Table 3, column 1 under "Jan", I want the average value of Jul to Dec
in Year 1 and Jan to June in Year 2.

For column 2 "Feb" in Table 3, it would be the average value of Aug to Dec
in Year 1 and Jan to Jul in Year 2.

For column 7 "Jul" in Table 3, it would be the average value of Jan to Dec
in Year 2.