To keep track of when records in a table are last modified or who made the modification you have two options:

1. Set default value under 'Edit as' settings.

To store login name of the user who made the change:

SESSION("UserID")

To store modification date and time:

Now()

Make sure to check off 'Apply on the Edit page as well' check box.

2. Add the following code to BeforeAdd/BeforeEdit events:

values("DateUpdated")=now()
values("UpdateBy")=SESSION("UserID")