CustomDateTimeConverter.cs

/// <summary>
/// Custom DateTime JSON serializer/deserializer
/// </summary>
public class CustomDateTimeConverter :  DateTimeConverterBase
{
    /// <summary>
    /// DateTime format
    /// </summary>
    private const string Format = "dd. MM. yyyy HH:mm";

    /// <summary>
    /// Writes value to JSON
    /// </summary>
    /// <param name="writer">JSON writer</param>
    /// <param name="value">Value to be written</param>
    /// <param name="serializer">JSON serializer</param>
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteValue(((DateTime)value).ToString(Format));
    }

    /// <summary>
    /// Reads value from JSON
    /// </summary>
    /// <param name="reader">JSON reader</param>
    /// <param name="objectType">Target type</param>
    /// <param name="existingValue">Existing value</param>
    /// <param name="serializer">JSON serialized</param>
    /// <returns>Deserialized DateTime</returns>
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.Value == null)
        {
            return null;
        }

        var s = reader.Value.ToString();
        DateTime result;
        if (DateTime.TryParseExact(s, Format, CultureInfo.InvariantCulture,DateTimeStyles.None, out result))
        {
            return result;
        }

        return DateTime.Now;
    }
}

usage.cs

[JsonConverter(typeof(CustomDateTimeConverter))]
public DateTime? PurchaseDate { get; set; }