FCI L1C reader overwrites units attribute on time metadata, making the data hard to interpret

[gerritholl]

Describe the bug

When loading time data with the FCI L1C NC reader, such as vis_06_time, the time attribute says s. In the original NetCDF files, it says seconds since 2000-01-01 00:00:00.0. Without looking at the original file, the user cannot interpret the float data in vis_06_time.

To Reproduce

import netCDF4
import hdf5plugin
from satpy import Scene
sc = Scene(filenames={"fci_l1c_nc": ["/media/nas/x23352/MTG/FCI/L1c-cases/202601051630/W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-1C-RRAD-FDHSI-FD--CHK-BODY--DIS-NC4E_C_EUMT_20260105164035_IDPFI_OPE_20260105163819_20260105163855_N_JLS_O_0100_0036.nc"]})
sc.load(["vis_06_time"])
print(sc["vis_06_time"].attrs["units"])

nc = netCDF4.Dataset("/media/nas/x23352/MTG/FCI/L1c-cases/202601051630/W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-1C-RRAD-FDHSI-FD--CHK-BODY--DIS-NC4E_C_EUMT_20260105164035_IDPFI_OPE_20260105163819_20260105163855_N_JLS_O_0100_0036.nc")
print(nc["time"].units)

Expected behavior

I expect the units are the same:

seconds since 2000-01-01 00:00:00.0
seconds since 2000-01-01 00:00:00.0

Actual results

Satpy turns it into s:

s
seconds since 2000-01-01 00:00:00.0

Environment Info:

• Satpy Version: v0.60.0

Additional context

I guess this happens because s is explicitly set as a unit in the YAML config:

satpy/satpy/etc/readers/fci_l1c_nc.yaml

Line 797 in 072b270

units: s

I have not tested what happens if this field is removed or changed.

[ameraner]

Thanks for the issue, good point!

[djhoese]

Side note: The next release of xarray will no longer decode timedelta fields from NetCDF by default. I've seen this with variables with attributes "units": "seconds" and old xarray would produce a np.timedelta64 but now in new versions it is just an integer array (the type of the in-file data).

[gerritholl]

I seem to recall we discussed decoding FCI time variables and decided against it. Changing that (by default) could be considered, but it would break backward compatibility. It is beyond the scope of this issue.