Time values for `phalf` and `pful` are not centered.

[chengzhuzhang]

example code to reproduce:

import xarray as xr

fn1 = 'https://esgf-data2.llnl.gov/thredds/dodsC/user_pub_work/CMIP6/CMIP/E3SM-Project/E3SM-2-0/historical/r1i1p1f1/Amon/pfull/gr/v20220830/pfull_Amon_E3SM-2-0_historical_r1i1p1f1_gr_185001-189912-clim.nc'
ds1= xr.open_dataset(fn1, chunks={"time": 4} )
ds1.pfull.time

Results of the time values:

xarray.DataArray'time'time: 600
array([cftime.DatetimeNoLeap(1850, 2, 1, 0, 0, 0, 0, has_year_zero=True),
cftime.DatetimeNoLeap(1850, 3, 1, 0, 0, 0, 0, has_year_zero=True),
cftime.DatetimeNoLeap(1850, 4, 1, 0, 0, 0, 0, has_year_zero=True), ...,
cftime.DatetimeNoLeap(1899, 11, 1, 0, 0, 0, 0, has_year_zero=True),
cftime.DatetimeNoLeap(1899, 12, 1, 0, 0, 0, 0, has_year_zero=True),
cftime.DatetimeNoLeap(1900, 1, 1, 0, 0, 0, 0, has_year_zero=True)],
dtype=object)