Need timestamp function that will change within a transaction

I'd like to find a timestamp function that WILL change within a transaction.

This function will return to me a 15 digit BIGINT number in base10:

   SELECT FLOOR(EXTRACT(EPOCH FROM NOW()) * 100000)::bigint;

The problem is that NOW() does not change within a transaction and so I
keep getting the same value.  Is there a function that will return a
timestamp that is NOT consistent within a transaction?

-- Dante