time range of the current dataset
        
        :keyword flag: use a flag array to know the time range of an indexed slice of the object

def time_range(self,flag=None):
        '''
        time range of the current dataset
        
        :keyword flag: use a flag array to know the time range of an indexed slice of the object
        '''
        if self.count==0: return [[None,None],[None,None]]
        
        if flag is None : return cnes_convert([self.date.min(),self.date.max()])
        else : return cnes_convert([self.date.compress(flag).min(),self.date.compress(flag).max()])