issues: 1925758025

What happened?

This is my example, that doesn't work since v2023.09.0:

```python import numpy as np import pandas as pd import xarray as xr

time = pd.date_range("1970-01-01", "1970-01-31", freq="6h") ds = xr.Dataset(coords=dict(time=time))

units = "days since 1960-01-01 00:00:00" calendar = "gregorian" encoding = dict(time=dict(units=units, calendar=calendar, dtype=np.dtype("float64")))

ds.to_netcdf("test.nc", encoding=encoding)

! ncdump -v time test.nc ```

This gives the following output:

``` netcdf test { dimensions: time = 121 ; variables: double time(time) ; time:_FillValue = NaN ; time:units = "days since 1960-01-01" ; time:calendar = "gregorian" ; data:

time = 3653, 3653, 3653, 3653, 3654, 3654, 3654, 3654, 3655, 3655, 3655, 3655, 3656, 3656, 3656, 3656, 3657, 3657, 3657, 3657, 3658, 3658, 3658, 3658, 3659, 3659, 3659, 3659, 3660, 3660, 3660, 3660, 3661, 3661, 3661, 3661, 3662, 3662, 3662, 3662, 3663, 3663, 3663, 3663, 3664, 3664, 3664, 3664, 3665, 3665, 3665, 3665, 3666, 3666, 3666, 3666, 3667, 3667, 3667, 3667, 3668, 3668, 3668, 3668, 3669, 3669, 3669, 3669, 3670, 3670, 3670, 3670, 3671, 3671, 3671, 3671, 3672, 3672, 3672, 3672, 3673, 3673, 3673, 3673, 3674, 3674, 3674, 3674, 3675, 3675, 3675, 3675, 3676, 3676, 3676, 3676, 3677, 3677, 3677, 3677, 3678, 3678, 3678, 3678, 3679, 3679, 3679, 3679, 3680, 3680, 3680, 3680, 3681, 3681, 3681, 3681, 3682, 3682, 3682, 3682, 3683 ; } ```

It seems like the subdaily fraction is truncated. Note, that this does not happend, if i set the units to the start of the time range, e.g., units = "days since 1970-01-01 00:00:00". This results correctly in

``` netcdf test { dimensions: time = 121 ; variables: double time(time) ; time:_FillValue = NaN ; time:units = "days since 1970-01-01" ; time:calendar = "gregorian" ; data:

time = 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.25, 3.5, 3.75, 4, 4.25, 4.5, 4.75, 5, 5.25, 5.5, 5.75, 6, 6.25, 6.5, 6.75, 7, 7.25, 7.5, 7.75, 8, 8.25, 8.5, 8.75, 9, 9.25, 9.5, 9.75, 10, 10.25, 10.5, 10.75, 11, 11.25, 11.5, 11.75, 12, 12.25, 12.5, 12.75, 13, 13.25, 13.5, 13.75, 14, 14.25, 14.5, 14.75, 15, 15.25, 15.5, 15.75, 16, 16.25, 16.5, 16.75, 17, 17.25, 17.5, 17.75, 18, 18.25, 18.5, 18.75, 19, 19.25, 19.5, 19.75, 20, 20.25, 20.5, 20.75, 21, 21.25, 21.5, 21.75, 22, 22.25, 22.5, 22.75, 23, 23.25, 23.5, 23.75, 24, 24.25, 24.5, 24.75, 25, 25.25, 25.5, 25.75, 26, 26.25, 26.5, 26.75, 27, 27.25, 27.5, 27.75, 28, 28.25, 28.5, 28.75, 29, 29.25, 29.5, 29.75, 30 ; } ```

What did you expect to happen?

I expect subdaily frequencies to be encoded correctly also if the units startdate is different from the startdate of the time axis, e.g. v2023.08.0 correctly preserves fractions:

``` netcdf test { dimensions: time = 121 ; variables: double time(time) ; time:_FillValue = NaN ; time:units = "days since 1960-01-01" ; time:calendar = "gregorian" ; data:

time = 3653, 3653.25, 3653.5, 3653.75, 3654, 3654.25, 3654.5, 3654.75, 3655, 3655.25, 3655.5, 3655.75, 3656, 3656.25, 3656.5, 3656.75, 3657, 3657.25, 3657.5, 3657.75, 3658, 3658.25, 3658.5, 3658.75, 3659, 3659.25, 3659.5, 3659.75, 3660, 3660.25, 3660.5, 3660.75, 3661, 3661.25, 3661.5, 3661.75, 3662, 3662.25, 3662.5, 3662.75, 3663, 3663.25, 3663.5, 3663.75, 3664, 3664.25, 3664.5, 3664.75, 3665, 3665.25, 3665.5, 3665.75, 3666, 3666.25, 3666.5, 3666.75, 3667, 3667.25, 3667.5, 3667.75, 3668, 3668.25, 3668.5, 3668.75, 3669, 3669.25, 3669.5, 3669.75, 3670, 3670.25, 3670.5, 3670.75, 3671, 3671.25, 3671.5, 3671.75, 3672, 3672.25, 3672.5, 3672.75, 3673, 3673.25, 3673.5, 3673.75, 3674, 3674.25, 3674.5, 3674.75, 3675, 3675.25, 3675.5, 3675.75, 3676, 3676.25, 3676.5, 3676.75, 3677, 3677.25, 3677.5, 3677.75, 3678, 3678.25, 3678.5, 3678.75, 3679, 3679.25, 3679.5, 3679.75, 3680, 3680.25, 3680.5, 3680.75, 3681, 3681.25, 3681.5, 3681.75, 3682, 3682.25, 3682.5, 3682.75, 3683 ; } ```

Minimal Complete Verifiable Example

```Python import numpy as np import pandas as pd import xarray as xr

time = pd.date_range("1970-01-01", "1970-01-31", freq="6h") ds = xr.Dataset(coords=dict(time=time))

units = "days since 1960-01-01 00:00:00" calendar = "gregorian" encoding = dict(time=dict(units=units, calendar=calendar, dtype=np.dtype("float64")))

ds.to_netcdf("test.nc", encoding=encoding)

! ncdump -v time test.nc ```

MVCE confirmation

• [X] Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• [X] Complete example — the example is self-contained, including all data and the text of any traceback.
• [X] Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• [X] New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

This is still an issue in the current main.

Environment

INSTALLED VERSIONS ------------------ commit: None python: 3.10.12 | packaged by conda-forge | (main, Jun 23 2023, 22:39:40) [Clang 15.0.7 ] python-bits: 64 OS: Darwin OS-release: 22.6.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: None LOCALE: (None, 'UTF-8') libhdf5: 1.14.2 libnetcdf: 4.9.2 xarray: 2023.9.1.dev12+gd5f17858 pandas: 2.1.1 numpy: 1.24.4 scipy: 1.11.3 netCDF4: 1.6.4 pydap: installed h5netcdf: 1.2.0 h5py: 3.9.0 Nio: None zarr: 2.16.1 cftime: 1.6.2 nc_time_axis: 1.4.1 PseudoNetCDF: 3.2.2 iris: 3.7.0 bottleneck: 1.3.7 dask: 2023.9.3 distributed: 2023.9.3 matplotlib: 3.8.0 cartopy: 0.22.0 seaborn: 0.13.0 numbagg: 0.2.2 fsspec: 2023.9.2 cupy: None pint: 0.20.1 sparse: 0.14.0 flox: 0.7.2 numpy_groupies: 0.10.2 setuptools: 68.2.2 pip: 23.2.1 conda: None pytest: 7.4.2 mypy: None IPython: 8.16.1 sphinx: None