I'm not near a copy of Pegasus Mail at the moment.

You can change the sort order of the folder from the Folder menu.  Change it to Date.  This should work even if the Date column is missing, although you probably won't be able to validate that it worked.  ;-) 

If you really do want to see your Date/Time column again, try holding down the Ctrl key and pressing the "-" key on the keypad (not the one near the Backspace key).  You may have to do this with the folder opened up in a classic folder window rather than preview mode (I don't use that mode so cannot be sure whether it works in preview mode).  Also, as a sort of "reset the whole UI to square one again" solution, I believe this info is stored in either the FOLSTATE.PM file or the STATE.PMJ file.  Both are located in your home mailbox directory.  You can try renaming both of these files while Pegasus Mail is not running and then start Pegasus Mail to see if all is well.

In Preview mode, looking at all the messages listed in in the right pane, the "Date" column is missing. "From" and "Subject" columns are there, but I the "Date" is no longer showing and there is no scroll bar showing.

 How can I get the messages listed by "Date" once again. It used to show up.

 I don't know if this is related, but the message Preview mode where I used to be able to look at the contents of each message when I clicked on it in the list has not been working for a long time. Any suggestion on how to get that working again,as well.

 Thanks. I've used Pegasus Mail for more than a decade...