Avoid Converting DateTimeOffset to UTC time in Azure EventGrid

I've been testing out Azure's EventGrid and one of the things I'm trying to do is send an event where part of the PayLoad is a DateTimeOffset field. I'm publishing the event using the standard EventGridClient:

using (var client = new EventGridClient(topicCredentials))
{
    await client.PublishEventsAsync(topicHostname, eventGridEvents).ConfigureAwait(false);
}

I've fired up Fiddler and verified that the date portion is getting serialized as I would expect: "CheckInDate": "2018-12-26T14:47:42.8-08:00"

But when my subscribed Azure Functions listener receives the HttpTrigger and reads the message's content, the same item has somehow been formatted as UTC: "CheckInDate": "2018-12-26T22:47:42.8+00:00".

Is there a setting or something I need to enable to get it to preserve the original offset?