Lookup and Display of Matched Cell Data?

Hello,
I have a two column list of "Days" and "Dollars." Days increase, dollars decrease. The amount the dollars decrease changes as you play around with some variables determined elsewhere. It's basically a quick forecasting tool, and allows you to fiddle with numbers to see when you'll get to a particular dollar amount.

Consequently, I would like a way to have a cell display the "Day" that closest matches a predetermined dollar amount defined in another cell. Rough is OK, it doesn't matter if it is slightly over or slightly under.

In more real world terms, suppose I have days 1-365 in column A, and dollar amounts starting at 5,000 and ending around 1,000 in column B descending. I'd like to be able to put "$3384" in a cell, and have another cell display the Day that is more or less closest to that amount.

Is this possible in real time? (ie, something that changes as you change numbers on the worksheet?)

Thanks,
Nathan