I have a table with 3 fields [DateIn] [Rates] and [LastModified]

DateIn default value it is set to Now() in the table

When a user enters a new record he is required to enter a Rate only. The
system will populate the filed DateIn automatically and at this stage the
LastModified field NOT have any value since the initial [Rate] has not been
modified yet.

Then, the user can open up a form and update the [Rate], if needed. This is
where I need to start to keep track of the DateModified only after the
initial Rate changes not the initial setup.

I tried to set me.LastModified = Now() in before update, after update, on
change, on dirty and nothing seem to work as I expect meaning the system
enter the LastModified date even on the initial entry (the same form is used
to enter new records). Any suggestion?