JSON timestamp to insert into time series database

Hi! /action/insertOne works like a charm for standard data, but we are struggling to do the same for a time series database.
Without fail, we are getting

[‘t’ must be present and contain a valid BSON UTC datetime value]

no matter how we are trying to format the timestamp.

The only API doc reference made to timestamps we find is:
{ "T": 1565545664, "I": 1}

Would be grateful for any pointers into how to approach this!