Bill, thanks for your reply! However, what you've done is something I already have, but not what I'm looking for. (It is useful though, to a degree.)
Let me attempt to clarify...
Two libraries: Accounts and Sales.
Sales entries are already linked in Accounts, so you can initiate new Sales entries while in the Accounts library, and you can even see all the sales for a particular account in the Tracking page of the Accounts library. I already have all of that set up.
What I am trying to do is take the date of the last new entry in Sales for a given Account and have that date show up in a SEPARATE field on a different page of the Accounts library (the Main page). So without leaving the Accounts library, and not going to the Sales Tracking page (which shows ALL the linked entries), I can, at quick glance, on the Main page of the library, see the most recent sale date.
This has the benefit of convenience (shows up at a glance on the first screen amongst other most important info for that acct), as well as giving me a field that I can use to sort with or base reports on, when I export my data from the Accounts library to a spreadsheet, to do various reports for the office.
So, in summary: when a new linked Sales library entry is created, I'd like it to update the date in the "Last Sale" field in the Accounts library.