timeexample.py

import datetime

def period_interval(date, period, n0, n1):
    """Return a 2-tuple of [start_date, end_date],

    date: base date from which to calculate forwards/backwards
    period: timedelta
    n0: int, starting period number
    n1: int, ending period number"""
    return [date + (period * n0),
            date + (period * n1)]


date = datetime.datetime(2015, 11, 18)
period = datetime.timedelta(days=14)
interval = period_interval(date, period, -1, 0)

print(date)
print(period)
print(interval)

# dev01:~$ python timeexample.py
# 2015-11-18 00:00:00
# 14 days, 0:00:00
# [datetime.datetime(2015, 11, 4, 0, 0), datetime.datetime(2015, 11, 18, 0, 0)]