timedelta_to_float fails on `cftime._cftime.Datetime360Day` calendars

[erikvansebille]

While working on #2483, I ran into an issue in our timedelta_to_float function in time.py, which can't handle the array below

[datetime.timedelta(0) datetime.timedelta(days=1)
 datetime.timedelta(days=2) datetime.timedelta(days=3)
 datetime.timedelta(days=4) datetime.timedelta(days=5)
 datetime.timedelta(days=6) datetime.timedelta(days=7)
 datetime.timedelta(days=8) datetime.timedelta(days=9)]

The following test breaks

data_folder = parcels.download_example_dataset("MITgcm_example_data")
ds_fields = xr.open_dataset(data_folder / "mitgcm_UV_surface_zonally_reentrant.nc")

# t = ds_fields["time"].values
# secs = np.array([(ti - t[0]).total_seconds() for ti in t])
# td_ns = np.rint(secs * 1e9).astype("int64").astype("timedelta64[ns]")
# ds_fields = ds_fields.assign_coords(time=td_ns)

coords = ds_fields[["XG", "YG", "Zl", "time"]]
ds_fset = convert.mitgcm_to_sgrid(fields={"U": ds_fields.UVEL, "V": ds_fields.VVEL}, coords=coords)
fieldset = FieldSet.from_sgrid_conventions(ds_fset)

npart = 10
lon = [24e3] * npart
lat = np.linspace(22e3, 1950e3, npart)

pset = parcels.ParticleSet(fieldset, lon=lon, lat=lat)
pset.execute(AdvectionRK4, runtime=np.timedelta64(5, "D"), dt=np.timedelta64(30, "m"))

with error

src/parcels/_core/particleset.py:463: in execute
    self._kernel.execute(self, endtime=next_time, dt=dt)
src/parcels/_core/kernel.py:256: in execute
    f(pset[evaluate_particles], self._fieldset)
src/parcels/kernels/_advection.py:61: in AdvectionRK4
    (u1, v1) = fieldset.UV[particles]
               ^^^^^^^^^^^^^^^^^^^^^^
src/parcels/_core/field.py:337: in __getitem__
    return self.eval(key.time, key.z, key.lat, key.lon, key)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/parcels/_core/field.py:322: in eval
    particle_positions, grid_positions = _get_positions(self.U, time, z, y, x, particles, _ei)
                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/parcels/_core/field.py:468: in _get_positions
    grid_positions.update(_search_time_index(field, time))
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/parcels/_core/index_search.py:89: in _search_time_index
    time_flt = timedelta_to_float(field.data.time.data - field.time_interval.left)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

dt = array([datetime.timedelta(0), datetime.timedelta(days=1),
       datetime.timedelta(days=2), datetime.timedelta(days=3...ays=6), datetime.timedelta(days=7),
       datetime.timedelta(days=8), datetime.timedelta(days=9)],
      dtype=object)

    def timedelta_to_float(dt: float | timedelta | np.timedelta64) -> float:
        """Convert a timedelta to a float in seconds."""
        print(dt)
        if isinstance(dt, timedelta):
            return dt.total_seconds()
        if isinstance(dt, np.timedelta64):
            return float(dt / np.timedelta64(1, "s"))
        if hasattr(dt, "dtype") and np.issubdtype(dt.dtype, np.timedelta64):  # in case of array
            return (dt / np.timedelta64(1, "s")).astype(float)
>       return float(dt)
               ^^^^^^^^^
E       TypeError: only length-1 arrays can be converted to Python scalars

src/parcels/_core/utils/time.py:168: TypeError

Note that the code does work with the commented-out lines (so updating ds_fields["time"]), which originally is

<xarray.DataArray 'time' (time: 10)> Size: 80B
array([cftime.Datetime360Day(2000, 1, 2, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 3, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 4, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 5, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 6, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 7, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 8, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 9, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 10, 0, 0, 0, 0, has_year_zero=True),
       cftime.Datetime360Day(2000, 1, 11, 0, 0, 0, 0, has_year_zero=True)],
      dtype=object)