Put your Code here or
try
Convert.ToDateTime(txtDate.Text);
And in Source you can try the following,-
<cc1:calendarextender id="CalendarExtender2" runat="server" targetcontrolid="txtPeriod" xmlns:cc1="#unknown">
Format="dd MMM yyyy">
</cc1:calendarextender>
<regularexpressionvalidator id="RVal" runat="server" controltovalidate="txtPeriod">
ErrorMessage="Invalid Format" ValidationExpression="^((31(?!\ (Feb(ruary)?|Apr(il)?|June?|(Sep(?=\b|t)t?|Nov)(ember)?)))|((30|29)(?!\ Feb(ruary)?))|(29(?=\ Feb(ruary)?\ (((1[6-9]|[2-9]\d)(0[48]|[2468][048]|[13579][26])|((16|[2468][048]|[3579][26])00)))))|(0?[1-9])|1\d|2[0-8])\ (Jan(uary)?|Feb(ruary)?|Ma(r(ch)?|y)|Apr(il)?|Ju((ly?)|(ne?))|Aug(ust)?|Oct(ober)?|(Sep(?=\b|t)t?|Nov|Dec)(ember)?)\ ((1[6-9]|[2-9]\d)\d{2})$"
ValidationGroup="Grp"></regularexpressionvalidator>