Search On Combination Of Two Columns

I have an ASP page where I am building a query to get the data from the
table. I am using Microsoft Access and ASP

The table has three columns Id, StartDate, EndDate.

For each record the users only enter either the startdate or enddate and not
both.

When i am writing my querystring I want to sort my results by date but the
date should be a combination of StartDate, EndDate

I tried something like this

SELECT * FROM Table ORDER BY NZ(StartDate,EndDate)

This works in the database directly but on the ASP page it does not allow
the NZ expression.