For example, if the post doesn't show up for two days, doesn't the postThe Date field in the message header is unaltered. But what your user interface shows you could be a different story.
Is this the way groups.io works? Or is the date and time stamp actuallyHere, and on Yahoo, the message you receive in email shows the origin date/time - when the sender sent it (with most email systems). Some user interfaces may give you the option of displaying the received date/time instead.
Here, in the Thread View, the Messages view, and the opened message, the Approved date/time is shown. You can find the origin Date/Time in the View Source of the message. If the message did not require moderation I'm not exactly sure what date/time is used, but it is likely to be within seconds of origin date/time.
In Y!Groups the Topics and Messages lists only show the date, not the time; so I'm not sure. The opened message shows the origin date/time.