时间和日期转换

> 当月的最后一天的日期

calendar.monthrange = monthrange(year, month)
    Return weekday (0-6 ~ Mon-Sun) and number of days (28-31) for
    year, month.


>>> def get_month_last_day(year,month):
...   return "%d-%d-%d"%(year,month,calendar.monthrange(year,month)[-1])
...
>>> get_month_last_day(2011,11)
'2011-11-30'