Unix date serialization

public class UnixDateTimeConverter : DateTimeConverterBase
    {
        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            if (reader.TokenType != JsonToken.Integer)
            {
                throw new Exception(
                    String.Format("Unexpected token parsing date. Expected Integer, got {0}.",
                    reader.TokenType));
            }

            var ticks = (long)reader.Value;

            var date = new DateTime(1970, 1, 1);
            date = date.AddSeconds(ticks);

            return date;
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            long ticks;
            if (value is DateTime)
            {
                var epoc = new DateTime(1970, 1, 1);
                var delta = ((DateTime)value) - epoc;
                if (delta.TotalSeconds < 0)
                {
                    throw new ArgumentOutOfRangeException(
                        "Unix epoc starts January 1st, 1970");
                }
                ticks = (long)delta.TotalSeconds;
            }
            else
            {
                throw new Exception("Expected date object value.");
            }
            writer.WriteValue(ticks);
        }
    }