diff String.Isnullorempty & String.Notnullorempty

public Boolean Validdata()
    {
        Boolean Valid = true;
        if (string.IsNullOrEmpty(txtDate.Text))
        {
            Page.ClientScript.RegisterStartupScript(this.GetType(), "Hello", "javascript:alert('Date Not Given')", true);
            Valid = false;
        }
        if (string.IsNullOrEmpty(txtStartTime.Text))
        {
            Page.ClientScript.RegisterStartupScript(this.GetType(), "Hello", "javascript:alert('StartTime Not Given')", true);
            Valid = false;
        }
 return true;
    }