Also the manual states that I should be able to create it either using the timestamp (is a valid UTC timestamp, that I got it from the VirusTotal API report) or a JScript Date object, but neither gives the correct date.
Looks like this was left out of the docs (although mentioned in the release notes for 13.2.1); use the SetTime() method to set the timestamp using the epoch time in milliseconds (so multiply by 1000 to use a unix epoch timestamp). Works the same as the JScript Date.SetTime and Date.GetTime methods.