Program.cs

// https://groups.google.com/d/msg/jayrock/t5GI2ta5Pow/p3_CtsWFiPIJ

using System;
using System.Globalization;
using Jayrock.Json;
using Jayrock.Json.Conversion;
using Jayrock.Json.Conversion.Converters;

sealed class TimeSpanImporter : ImporterBase
{
    public TimeSpanImporter() : base(typeof(TimeSpan)) { }

    protected override object ImportFromString(ImportContext context, JsonReader reader)
    {
        return TimeSpan.Parse(reader.ReadString(), CultureInfo.InvariantCulture);
    }
}

public class ETest
{
    public TimeSpan tm { get; set; }
}

static class Program
{
    static void Main()
    {
        var context = JsonConvert.CreateImportContext();
        context.Register(new TimeSpanImporter());
        const string json = "{\"tm\":\"11:28:47\"}";
        var eTest = context.Import<ETest>(JsonText.CreateReader(json));
        Console.WriteLine(eTest.tm);
    }
}