In ArcMap 10.4 I have the following code in Pre-Logic Script Code Section in Field Calculator with python option selected and show code block:
from datetime import datetime def myfunction(date_text): try: if date_text != datetime.strptime(date_text, "%m/%d/%Y").strftime('%H:%M:%S'): return date_text except ValueError: return None
And in bottom section for Field I want to update which is LAST_DATE_:
The code above works by instead checking if the value is a Time value. If it is then update the value works. However you will have to switch %d/%m/%Y to cover all your values. For some reason it would not add single digit day values.