Date Logic in lookups

[admin]

Thanks to all for the responses on my last post.

My final question for today relates to date logic in lookups. Is there a way to lookup a record by effective date in a hashed lookup? Ive tried writing in date formulas to no avail. Given the following records:

1 ABC 1/1 3/31
2 ABC Co. 4/1

I want to match on the correct invoicing coming in. So if I have an invoice dated Jan 12, I want record 1, but if my incoming record is dated May 15, I want the second record. Im trying to avoid outer joins in my source. Is there a way to do this directly in the transformation stage, or am I better off just doing the outer join?

thanks again.

jason