date diff

This should be easy. But I cannot get it right.

I have 2 date fields. General Date - Format m/d/yy hh:mm
I want to create a query that will return the difference between the dates in HH:MM

So I figured this would work

SELECT DateDiff(hh,Field2,Field1) AS 'diff'
FROM dateTest;

But it prompts me for an hh value.

Can anyone help me out with this.

thanks