Has anybody used date_happened for the datadog logstash output plugin

I am unable to find an example config.
The documentation says string.
I tried
date_happened => "1474319717"

that didn't work

I turned up verbosity on logstash
I see that when I don't specify date_happened in my filter, that the plugin uses a number (not a string):

DataDog event {:dd_event=>{"title"=>"short test title", "text"=>"arbitrary text for testing, using warning for alert_type, 5:04:03", "source_type_name"=>"my apps", "alert_type"=>"warning", "date_happened"=>1474320888},

This request succeeded with a 202 response code.

so I tried using a number in my plugin

date_happened => 1474320888

but the plugin converts my number to a string and sends date_happenned as a string to datadog.

This request fails with a 400.

So, how can I specify date_happened?

Yes, I realize that if I leave it off, that the plugin will use "now".
However, I don't want "now".
I want to be able to specify the time.

I even tried this hack with no joy (still get a 400).

filter {
mutate {
rename => { "[message]" => "[@metadata][incoming-message]" }
add_field => { "[epoch-start]" => "1474319717" }

Set @TIMESTAMP from epoch-start

date { match => [ "[epoch-start]", "UNIX_MS" ] }