pytz and astimezone() cannot be applied to a naive datetime

For pytz timezones, use their .localize() method to turn a naive datetime object into one with a timezone:

start_date = local_tz.localize(start_date)

For timezones without a DST transition, the .replace() method to attach a timezone to a naive datetime object should normally also work:

start_date = start_date.replace(tzinfo=local_tz)

See the localized times and date arithmetic of the pytz documentation for more details.