Algo to parse periods of time.

gistfile1.py

#!/usr/bin/python2.6

import datetime

dates = [ 
        datetime.date(2011, 01, 01),
        datetime.date(2011, 01, 11),
        datetime.date(2011, 01, 15),
        datetime.date(2011, 01, 25),
        datetime.date(2011, 01, 30),
        datetime.date(2011, 02, 10),
        datetime.date(2011, 02, 15),
        datetime.date(2011, 02, 20),
        datetime.date(2011, 02, 28),
        datetime.date(2011, 03, 10),
        datetime.date(2011, 03, 15),
        datetime.date(2011, 03, 18),
        datetime.date(2011, 03, 18),
        datetime.date(2011, 03, 18),
       ]   

current_date = dates[0]
period_num = 0 
for date in dates:
    delta = date - current_date
    if delta.days >= 21: 
        print 'Period of 21 days reached (%d days)' % delta.days
        current_date = date
        period_num += 1
        print '## Period %d ##' % period_num
        print ''
    else: 
        print 'Delta: %d . Moving to next period' % delta.days