Updating User expiration dates using API

I'm using the "update users API" to change the expiration date for a batch of users.

I am getting a response when I send the post, and it is actually changing the users. But instead of changing to the date I have set in the request, it's changing to "null/never." I've looked at the documentation surrounding the date format (although it's somewhat limited) and I even tried setting one user to the date I wanted, then copying the value from a "get user" response and pasting it into my body, but the same thing keeps happening.

I'm sure I'm missing something obvious, anyone have any ideas?

Here is my post body:

{

"action": "createOrUpdate",

"input": [

 {

"expiresAt": "2023-12-31T06:00:00.000t+0000"

 }

 ]

}