Trigger on Insert to Update only newly inserted fields?

I have a On Insert Trigger that updates one of the columns in that same
table.

Is there a way for the trigger to run only for the newly inserted records?
Instead of all records in the database?

E.g.:
ID Start_Date End_Date
001 08-01-2006
002 08-02-2006

On Insert/Update, Update End_Date=now().
I want that to run only on new records.or the updated
record. How can I do this?

Thank you so much.