All Python Function is not working in Script task

dateutil is not part of the python standard library, so it cannot be called from the Script.

Do something like

from datetime import datetime
INCR_LOAD_START_TIME = "2017-08-27T00:00:00"
start = datetime.strptime(INCR_LOAD_START_TIME, "%Y-%m-%dT%H:%M:%S")

Look at python - string to datetime with fractional seconds, on Google App Engine - Stack Overflow if you want to handle the milliseconds