setDayOfMonth raises ValueError if argument does not fall in 
day range for month:

>>> from xix.utils.timetool import setDayOfMonth
>>> from time import strptime, strftime, mktime, tzset
>>> from datetime import datetime
>>> t = strptime('2005-02-01', '%Y-%m-%d')
>>> dt = datetime.fromtimestamp(mktime(t))
>>> try:
...     dt = setDayOfMonth(dt, 29)
... except ValueError:
...     print 'scheisse!'
...
scheisse!
>>> try:
...     dt = setDayOfMonth(dt, 0)
... except ValueError:
...     print 'scheisse!'
...
scheisse!


