When using the CalendarExtender from the Ajax Control Toolkit to add pop up functionality to a text box, there are a couple of things you need to do to ensure that the date format in the text box is correct for your culture.
Firstly, add Culture and UI Culture information to your web.config file. In short add an element to the <system.web> section. In my case (UK English) I add <globalization uiCulture="en" culture="en-GB" /> to this element.
<system.web> <globalization uiCulture="en" culture="en-GB" /> </system.web>
Secondly, you need to instruct the script manager to actually take note of this. You do this by setting the EnableScriptGlobalization and EnableScriptLocalization to true as properties of the ScriptManager.
<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" EnablePartialRendering="true" EnableScriptGlobalization="true" EnableScriptLocalization="true"> </cc1:ToolkitScriptManager>
Done deal <grin>