Sending a unix timestamp in a HTTP post

I would like to send a timestamp in a HTTP post with a body type of JSON. Here is the pseudocode I would like {"rack":%currentRack%, "timeStamp":%unixTimeStamp%}. Is there some combination of %date% and %time% that can do this?

Corollary 1: is the a way to format %date% the way you can %time%
Corollary 2: Where is the documentation of the variables you can use such as %date%.

Thank you in advance.

Rule Machine has a built-in option when you use the "Set Variable" action to set a variable to epoch/Unix time, in either second or millisecond format. You'll just need to create a local (or hub) variable of type "number" to hold the result of this value. The built-in %date% and %time% variables won't necessarily help you, both because it would be difficult to convert them to this format but also because they refer to the time of the last trigger event, not necessarily the current time. The %now% variable is closer to what you're looking for but is not formatted as Unix time.

For the most part, these are documented at the places in the UI where you can use them, notably the "Send/speak a message" action. However, there are now lots of places in the UI where you can use these or any local or hub variable (in the same format), and these are not all noted. Perhaps it could be documented elsewhere.