A number of months ago, someone very graciously designed the following formula:
=(SUMPRODUCT(LOOKUP(ROW(INDIRECT(C$4&":"&EOMONTH(C$4,0)))+6,'CJNH 2012'!$3:$3,'CJNH 2012'!155:155))/7)

This is used to calculate monthly payroll accruals. So, in row 3 in CJNH 2012 there are dates, e.g. 2/4/12, 2/11/12, 2/18/12, 2/25/12, 3/3/12

If C4 = 2/1/2012, this formula will add 4/7ths of the cell in row 155 sharing the same column as 2/4/12, the entirety of the cell in row 155 sharing the same column as 2/11/12, 2/18/12 and 2/25/12, and 3/7ths of the cell in row 155 sharing the same column as 3/3/12. This formula does the calculation perfectly, but I can't figure out why it works. And even though I was satisfied for a while that it does exactly as its supposed to do, I'm at the point now where I'd really like to know the logic behind it.