use Ajax Calender Extender Controller following way...its never loose value after post back of page
<asp:TextBox runat="server" ID="MyTextBoxFromDate"></asp:TextBox>
<ajax:CalendarExtender ID="MyTextBoxromDateCalendar" TargetControlID="MyTextBoxFromDate"
Format="dd/MM/yyyy" runat="server" />
<ajax:MaskedEditExtender ID="MaskedEditExtender1" runat="server" TargetControlID="MyTextBoxFromDate"
MaskType="Date" InputDirection="LeftToRight" MessageValidatorTip="true" Mask="99/99/9999">
</ajax:MaskedEditExtender>
you may add others required properties in above all controls as you your need...I have use just basic required properties