Using webhooks, I need a date field in milliseconds from the epoch...but I get MM-DD-YYYY. Is there an easy way to get milliseconds from the epoch UTC into a formula?

There are lots of people doing the reverse from the API calls but I need to try and get it into the "Message body" of a Webhook.  I can pick my date using [Invoice Date] but it spits out MM-DD-YYYY.  I also tried to create another field that is formulated from that date...but I don't see any of the options I'm looking for other than parsing the date...then brute forcing the seconds.