netcdftime does not support 'months since'

The netcdftime routines do not support times which have a 'months since' as a 
basedate. This seems not to be implemented so far. Would be great if it could 
be done

I wrote a small routine that uses the pylab num2date() function to convert 
times appropriately. For monthly data, the different calendars might not play a 
role.

Please find below two routines (as subs for one of my classes), which will do 
the job.

    def _get_date_from_month(self,nmonths):
        """
        calculate a datetime object for a time given in 'months since' a basedate
        The routine increments itteratively the number of months and returns a datetime object

        This is done for a *single* timestep!

        @param time_str: time string that specifies start date. Needs to contain 'months since'
        @type time_str: str

        @param nmonths: time as numeric value (number of months since basedate)
        @type nmonths: float or int

        @return: datetime object with actual date
        """

        if not 'months since' in self.time_str:
            print self.time_str
            raise ValueError, 'This routine is only for conversion of monthly data!'

        basedate = self.time_str.split('since')[1].lstrip()

        #--- start date
        start_date = pl.datestr2num(basedate)
        act_date = start_date*1.

        for i in xrange(int(nmonths)): #increment months
            d = pl.num2date(act_date)
            ndays = monthrange(d.year,d.month)[1] #number of days in current month
            act_date += ndays

        return pl.num2date(act_date)

#-----------------------------------------------------------------------

    def _convert_monthly_timeseries(self):
        """
        convert monthly timeseries to a daily timeseries
        """
        if self.calendar not in ['standard','gregorian',None]:
            print self.calendar
            raise ValueError, 'Not sure if monthly timeseries conversion works with this calendar!'

        newtime = [self._get_date_from_month(t) for t in self.time] #... estimate new time
        self.calendar = 'standard'
        self.time_str = 'days since 0001-01-01 00:00:00'
        self.time = pl.date2num(newtime)+1. #plus one because of the num2date() basedate definition

Hope this helps.

Best,
   Alex
Original issue reported on code.google.com by alexloew...@gmail.com on 16 Jul 2013 at 5:59
sodri126 / netcdf4-python

netcdftime does not support 'months since' #190
