Peter Kuhn has a short post that shows a straight-forward way to create a functioning simple DateTimePicker control.
Source: Mister Goodcat
A question that frequently surfaces on the Silverlight forums is the one after a DateTimePicker control. The Silverlight Toolkit has both a date picker and a time picker, but not the combination of both. The issue tracker of the Toolkit project on Codeplex lists that control among the Top-20 voted features. But when you ask about it, the answer usually is that it's so simple to build one yourself that it's not worth creating a dedicated control for that. However, when I looked at the source code of an attempted DateTimePicker control someone sent me in desperation, I realized that you can get lost in undesired behavior and unforeseeable problems with mutually triggering events easily.