Custom JSON converter for Noda Time

Firstly, this doesn't sound like the right thing to do in a JSON converter. If you're accepting user input directly in your JSON, that should be treated as a string, and parsed later, IMO. JSON is a machine-to-machine format, not a human-to-machine format. Assuming this is a web app, you might want to use moment.js to parse the data at the client and reformat it as ISO-8601. Alternatively, deserialize it as a string and then convert it in your server-side code.

Anyway, for a JSON converter you only need to implement IPattern<LocalDate> - you don't need to implement NodaPatternConverter<LocalDate> as that already exists. You just need:

var pattern = ...;
var converter = new NodaPatternConverter<LocalDate>(pattern);

Now, to implement your pattern, you probably want to actually create it out of existing patterns - write an implementation which delegates to one IPattern<LocalDate> after another until the result is a ParseResult<T> which is successful - or return the final unsuccessful ParseResult<T>. Note that ParseResult<T> isn't inaccessible - but you can't (currently) create your own instance of it. That's something I should probably address, but in this case you don't really need to.

The code you need already exists but isn't exposed - you want the Parse part of CompositePattern. To implement the Format part, you could just use the first of your patterns to format the value... if you even need to.