A dateutil error has appeared, due to updates? How to fix?

[tinnews at isbd.co.uk]

I have a python script which uses the dateutil module with the
following:-

    import sys
    import datetime
    import icalendar
    from dateutil.relativedelta import relativedelta

The section of code which uses relativedelta is as follows:-

        #
        #
        # If the event is a repeating event with a start date in the
        # past then we add the repeat interval until we find the next
        # occurrence
        #
        if eventDate < datetime.date.today():
            if event.has_key('RRULE'):
                freq = event.decoded('RRULE')['FREQ'][0]
                if event.decoded('RRULE').has_key('INTERVAL'):
                    interval = event.decoded('RRULE')['INTERVAL'][0]
                else:
                    interval = 1;
                if 'WEEKLY' == freq:
                    delta = relativedelta(weeks=+interval)
                if 'MONTHLY' == freq:
                    delta = relativedelta(months=+interval)
                if 'YEARLY' == freq:
                    delta = relativedelta(years=+interval)
                while eventDate < datetime.date.today():
                    eventDate += delta
            else:
                continue        # if it's a non-repeating event in the past


It used to work and all I have done meanwhile is to update my xubuntu
system as fixes are released.  I'm now getting the error:-

    Traceback (most recent call last):
      File "/home/chris/bin/calics.py", line 62, in <module>
        eventDate += delta
      File "/usr/lib/python2.7/dist-packages/dateutil/relativedelta.py", line 261, in __radd__
        day = min(calendar.monthrange(year, month)[1],
    AttributeError: 'module' object has no attribute 'monthrange'

Have I lost a module somewhere in the updates or has something in
python changed such that my code no longer works as it used to?

Can anyone help diagnose this please.

-- 
Chris Green