Excel 2010 index match - multiple criteria - excel-formula

Question

I am new to using index/match, and I'm facing some trouble. I have two worksheets, one with table act and the other with table wa. act contains 7,199 rows; wa has 25,099 rows.
I am trying to match order number and date in order to pull a document number from wa onto act. The order numbers will be an exact match, but the date on act is slightly less than the date on wa.
My formula is:
=INDEX(WA[BillingDocumentNumber],(MATCH([#[Customer PO Number]],WA[PO],0)+MATCH([#[GL Posting Date]],WA[CostPostedOn],-1)))
Depending on how I sort the wa data, I have gotten the following results:
494 document numbers returned (sometimes correct; sometimes pulling from the cell in wa directly beneath the cell that should be returned) with the rest #N/A;
approximately 1400 document numbers (all incorrect, I think) with the remaining being either #N/A or #REF
Can anyone help me? I have no practical knowledge of VBA, but I am the resident Excel "expert" (seems funny to me) in my company, so I am the person who is faced with the task of solving this problem... I have combed the existing forums, but I haven't found any that seem to provide a (non-VBA) solution for my problem. Any ideas would be greatly appreciated!
Thank you for your time.

Answer 1

Here is my untested proposed solution:
{=CONCATENATE(IFERROR(IF(AND(WA[PO]=#[Customer PO Number],WA[CostPostedOn]>#[GL Posting Date]),WA[BillingDocumentNumber],""),""))}
Entered as an Array Formula using CTRL + SHIFT + ENTER.
This will return in one text string all results that match the PO number and in which the act date is older than the wa date. You can modify your second and add a third conditional in the AND to create date ranges (ie WA[CostPostedOn]>=#[GL Posting Date]+7 or WA[CostPostedOn]<=#[GL Posting Date]-7).
As you are using arrays though there might be some performance issues when you scale this to 7,000+ formulas. Sadly you can't do a VLOOKUP with 2 conditionals.
Hope this helps. Cheers,