Nested classes will require a fully qualified reference, so you will need to add another xmlns entry to the xaml referencing your parent class namespace. e.g.
xmlns:ValueConverters="clr-namespace:YourAppName.MyPage;assembly=YourAppName"
In VS 2010 once you start typing auto-complete/intellisense should start listing your project namespaces*.
Then reference your value converter with
<ValueConverters:HahaConverter x:Key="dateConverter">
*Note: It is usually recomended to stick to the one-class one-file standard as tools, coders (and VS) cope better. You will likely wind up with a large collection of Value Converters for Silverlight, might as well start a library for them :)