ToString() -- 'No overload for method ToString takes 1 argument' error

Hi:

I'm trying to display a dateTime value in the format '07/31/06' but when I use the ToString() as follows I get the above errormessage, despite the fact that the documentation clearly shows that ToString( string ) exists. What's up with this

string timeStr = myDate.ToString("mm dd yy");

I also tried string timeStr = myDate.ToString( "d");

with the same result...

Thanks for all help!

John F.