ASP.NET Web API - ModelBinders

I think you don't need a model binder. Your approach is incorrect. The right approach for dates is using a client side globalization library like the globalize library to parse dates formatted in any language and transform them into JavaScript date objects. Then you can serialize your datascript data structures in JSON with the browser JSON.stringify, and this should work. It is better to use always standard formats for dates and to use a formatter instead than a model binder. Available formatters handle also TimeZones correctly, if you use the kind parameter of your C# DateTime objects to specify if the date time is expressed in local time or in UTC time.

You can register a model binder globally by implementing a ModelBinderProvider and inserting it into the list of services.

Example use in Global.asax:

GlobalConfiguration.Configuration.Services.Insert(typeof(ModelBinderProvider), 0, new Core.Api.ModelBinders.DateTimeModelBinderProvider());

Below is example code demonstrating a ModelBinderProvider and a ModelProvider implemenation that converts DateTime arguments in a culture aware manner (using the current threads culture);

DateTimeModelBinderProvider implementation:

using System;
using System.Web.Http;
using System.Web.Http.ModelBinding;

...

public class DateTimeModelBinderProvider : ModelBinderProvider
{
    readonly DateTimeModelBinder binder = new DateTimeModelBinder();

    public override IModelBinder GetBinder(HttpConfiguration configuration, Type modelType)
    {
        if (DateTimeModelBinder.CanBindType(modelType))
        {
            return binder;
        }

        return null; 
    }
}

DateTimeModelBinder implementation:

public class DateTimeModelBinder : IModelBinder
{
    public bool BindModel(HttpActionContext actionContext, ModelBindingContext bindingContext)
    {
        ValidateBindingContext(bindingContext);

        if (!bindingContext.ValueProvider.ContainsPrefix(bindingContext.ModelName) ||
            !CanBindType(bindingContext.ModelType))
        {
            return false;
        }

        bindingContext.Model = bindingContext.ValueProvider
            .GetValue(bindingContext.ModelName)
            .ConvertTo(bindingContext.ModelType, Thread.CurrentThread.CurrentCulture);

        bindingContext.ValidationNode.ValidateAllProperties = true;

        return true;
    }

    private static void ValidateBindingContext(ModelBindingContext bindingContext)
    {
        if (bindingContext == null)
        {
            throw new ArgumentNullException("bindingContext");
        }

        if (bindingContext.ModelMetadata == null)
        {
            throw new ArgumentException("ModelMetadata cannot be null", "bindingContext");
        }
    }

    public static bool CanBindType(Type modelType)
    {
        return modelType == typeof(DateTime) || modelType == typeof(DateTime?);
    }
}