The comments for SetTimer in TimerManager.h do not specify that the time is in seconds. This is a discrepancy from comments for Blueprint Set Timer nodes, which do specify that the time is in seconds.
The comment for InRate does not specify that the time is in seconds, as opposed to milliseconds.
The comment matches the comment for Blueprint nodes in specifying that the delay time is in seconds.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-44886 in the post.