Judy,

Maybe the reason it is set up like that is due to the various time
differences.

I suspect you're right. Relative time ("ago") is the same no matter what time zone the user is in.

Frances I can’t remember if I indicated in my profile on Yahoo Groups
what time zone I am in, I think I did, so my messages, etc. have the
date/time for my time zone.

Yes, the time zone listed in your Yahoo Account Info page is used to display date/time information throughout Yahoo - except on calendars (they have their own).

It used to be that you had to set the Account Info time zone correctly, but the last time I looked it is setting your time zone automatically based on location information.

I know any message I receive from the various Yahoo Groups, they show my
current time. I get my messages in my inbox of Outlook.

That's being done by Outlook. The Date field of email headers specify both a Date/Time and the UTC* offset of that Date/Time. With that information Outlook can convert the Date/Time to anyone's local time. For example, your first message in this thread had:

Date: Wed, 21 Jan 2015 19:23:32 -0500

The "-0500" is the offset from UTC of that date/time. I live in in UTC-0800, so my email software subtracts the difference in those offsets (3 hours) from that date/time and displays your messages as having been sent at "04:23 PM 1/21/2015" - in my local time.

-- Shal
* http://en.wikipedia.org/wiki/Coordinated_Universal_Time