How would I count unique dates associated with another specific value on a worksheet?

For instance, Lets say I want to count each day a particular person makes a sale.

Example:

Sales Person Sale ID Sales Date
Don 100 1/6/2014
Don 140 1/7/2014
Don 160 1/8/2014
Jack 101 1/6/2014
Jack 102 1/6/2014
Mike 90 1/5/2014
Mike 103 1/6/2014

So on another worksheet, I'd have:

Sales Person Days with Sale
Don 3
Jack 1
Mike 2

The Formula would go into the "Days with Sale" Column.

Any ideas?