slice object given a time range
        
        :parameter timerange: rime range to be used.

def time_slice(self,timerange,surf=False):
        '''
        slice object given a time range
        
        :parameter timerange: rime range to be used.
        '''
        if isinstance(timerange[0],str): trange = cnes_convert(timerange)[0]
        else: trange=timerange
        return self.slice('date',trange,surf=surf)