I have recently needed to use the Umbraco datePicker control in a usercontrol I was creating however I kept getting presented with an "Unknown server tag 'umbraco:datePicker" exception. After some digging around in the Umbraco Source it is confusingly not in the assembly I would have expected but it was in fact located in the cms.dll and not controls.dll. As a couple of other Umbracians posted this query a day or so after I found the solution I thought I would post it up for the benefit of others.
Simply include the following in your usercontrol:
<%@ Register TagPrefix="umbraco" Namespace="umbraco.controls" Assembly="cms" %>
and then use it as follows:
[code lang="csharp"]<umbraco:datePicker ID="datePicker1" runat="server" ShowTime="true"></umbraco:datePicker>[/code]
I hope this is of help to others and can be refactored asap to avoid any further confusion.