If you need something more configurable than @mythz's answer, you can force the serialization or deserialization of DateTimes to have a certain DateTimeKind
by overriding the DateTime and optionally DateTime? serialization and/or deserialization methods.
Force all serialized DateTimes to be interpreted as UTC
JsConfig<DateTime>.SerializeFn = time => new DateTime(time.Ticks, DateTimeKind.Local).ToString();
You can then take this one step farther and error on deserialization if the DateTime isn't in a specified format. I started using this when I wanted to force clients to specify the timezone in all requests, but not necessarily require that it always be Utc.
JsConfig<DateTime>.DeSerializeFn = time =>
{
if (!IsInCorrectDateFormat(time))
throw new System.Runtime.Serialization.SerializationException(BadDateTime);
return ServiceStack.Text.Common.DateTimeSerializer.ParseDateTime(time);
};