code: def default_validity_start():
      
      start = datetime.now() - timedelta(days=1)
      return start.replace(hour=0, minute=0, second=0, microsecond=0)
docs: Sets validity_start field to 1 day before the current date
      (avoids "certificate not valid yet" edge case).
  
      In some cases, because of timezone differences, when certificates
      were just created they were considered valid in a timezone (eg: Europe)
      but not yet valid in another timezone (eg: US).
  
      This function intentionally returns naive datetime (not timezone aware),
      so that certificates are valid from 00:00 AM in all timezones.