#!/usr/bin/env python
import os, os.path, sys, datetime
import xarray as xr

def year_shift(da, n, to_datetime=False):
    '''shift the time coords of da by n years'''
    da = da.copy()

    new_time = [t.replace(year=t.year+n) for t in da.time.values]
    da['time'] = new_time

    if to_datetime:
        new_time = [datetime.datetime(*t.timetuple()[0:6]) for t in da.time.values]
        da['time'] = new_time

    return da