Timestamp

Lots of examples

Timestamp
Track Changes
Vehicle Maintenance
Track Log On

Richard's video shows how to do this.  But there are a couple of caveats.  If the user changes the data and then changes it back to the original, the form's BeforeUpdate event will still run and consider that a change.  So it will update the time stamp when there is no change, which may not be what you want.  If the user backs out entirely (keeps pressing Esc), BeforeUpdate won't run.  But you don't want to count on your user doing that.

That's why I never used this method in my old job.  Depending on your business, modified dates could be very important information (certainly were in our case) -- so much so that a company-wide memo announcing the change could be necessary.  A time stamp field in this case would be redundant -- not to mention, may or may not work as I said above.

Another caveat in using record timestamp: you only know the date and time of the change, but you don't know WHAT FIELDS were changed.

In the industry I worked in (retail), we would get "order change documents" from customers who made changes to the orders they had given us.  This was to properly document the occasion so everyone involved would be on the same page.  In our database, there was NO time stamp field for every record -- because a single field could not tell the whole picture.  There could be changes to delivery dates, shipping carrier changes, terms changes, etc., that a time stamp field couldn't indicate.