DateTime Param For SP Causing BIG Headache...!!

I've got a stored procedure and one of the parameters is a DateTime.  But no matter what I do to the string that's passed into the form for that field, it doesn't like the format.  Here's my code:
 SqlConnection conn = new SqlConnection(KPFData.getConnectionString());
SqlCommand cmd = new SqlCommand("KPFSearchName", conn);
cmd.CommandType = CommandType.StoredProcedure;

SqlParameter param = cmd.Parameters.Add("@DOB", SqlDbType.SmallDateTime);
param.Direction = dir;
param.Value = txtDOB.Text;

// also have tried this:

param.Value = Convert.ToDateTime(txtDOB.Text);

// and

param.Value = Convert.ToDateTime(txtDOB.Text).ToShortDateString;

No matter what I do I always get a formatting error - either I can't convert the string to a DateTime, or the SqlParameter is in the incorrect format, or something along those lines.  I've spent a couple hours on this and hoping someone can point out my obvious mistake here...??

Thanks for your help!!

eddie