diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 80c3286c..dd83bea5 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [38, 39, 310]
+        python-version: [39, 310, 311]
 
     steps:
       - uses: actions/checkout@v4
@@ -16,6 +16,9 @@ jobs:
         uses: mamba-org/setup-micromamba@v1
         with:
           environment-file: ci/environment-py${{matrix.python-version}}.yml
+          cache-downloads: false
+          cache-environment: true
+          generate-run-shell: false
       - name: Install kerchunk
         shell: bash -l {0}
         run: |
diff --git a/ci/environment-py310.yml b/ci/environment-py310.yml
index 6c760a37..021b150f 100644
--- a/ci/environment-py310.yml
+++ b/ci/environment-py310.yml
@@ -1,7 +1,7 @@
 name: test_env
 channels:
   - conda-forge
-  - defaults
+  - nodefaults
 dependencies:
   - python=3.10
   - dask
diff --git a/ci/environment-py38.yml b/ci/environment-py311.yml
similarity index 93%
rename from ci/environment-py38.yml
rename to ci/environment-py311.yml
index 3d276036..d680ae71 100644
--- a/ci/environment-py38.yml
+++ b/ci/environment-py311.yml
@@ -1,9 +1,9 @@
 name: test_env
 channels:
   - conda-forge
-  - defaults
+  - nodefaults
 dependencies:
-  - python=3.8
+  - python=3.11
   - dask
   - zarr
   - xarray
diff --git a/ci/environment-py39.yml b/ci/environment-py39.yml
index f5f8f90d..e4ca09ad 100644
--- a/ci/environment-py39.yml
+++ b/ci/environment-py39.yml
@@ -1,7 +1,7 @@
 name: test_env
 channels:
   - conda-forge
-  - defaults
+  - nodefaults
 dependencies:
   - python=3.9
   - dask
diff --git a/kerchunk/combine.py b/kerchunk/combine.py
index 0aa4c68c..69ec3cc6 100644
--- a/kerchunk/combine.py
+++ b/kerchunk/combine.py
@@ -201,6 +201,7 @@ def append(
         ds = xr.open_dataset(
             fs.get_mapper(), engine="zarr", backend_kwargs={"consolidated": False}
         )
+        z = zarr.open(fs.get_mapper())
         mzz = MultiZarrToZarr(
             path,
             out=fs.references,  # dict or parquet/lazy
@@ -235,7 +236,7 @@ def append(
                     mzz.coos[var].add(value2)
 
             else:
-                mzz.coos[var] = set(ds[var].values)
+                mzz.coos[var] = set(z[var][:])
         return mzz
 
     @property
@@ -336,6 +337,8 @@ def _get_value(self, index, z, var, fn=None):
                     self.cf_units[var] = dict(units=units, calendar=calendar)
         else:
             o = selector  # must be a non-number constant - error?
+        if var in self.coo_dtypes:
+            o = np.array(o, dtype=self.coo_dtypes[var])
         logger.debug("Decode: %s -> %s", (selector, index, var, fn), o)
         return o
 
diff --git a/kerchunk/tests/test_combine.py b/kerchunk/tests/test_combine.py
index 3e08dc67..64e6e5c5 100644
--- a/kerchunk/tests/test_combine.py
+++ b/kerchunk/tests/test_combine.py
@@ -174,6 +174,19 @@
     b'1970-01-01T00:00:00"}',
 )
 
+tdata1 = xr.DataArray(
+    data=arr,
+    coords={"time": np.array([3])},
+    dims=["time", "x", "y"],
+    name="data",
+)
+xr.Dataset({"data": tdata1}).to_zarr("memory://cfstdtime3.zarr")
+fs.pipe(
+    "cfstdtime3.zarr/time/.zattrs",
+    b'{"_ARRAY_DIMENSIONS": ["time"], "units": "seconds since '
+    b'1970-01-01T00:00:00"}',
+)
+
 # cftime arrays - non standard
 tdata1 = xr.DataArray(
     data=arr,
@@ -345,6 +358,51 @@ def test_single_append(refs):
     assert z.time.values.tolist() == [1, 2, 3]
 
 
+@pytest.mark.parametrize("mapper", [{}, {"time": "cf:time"}])
+@pytest.mark.parametrize("dtype", [{"time": "M8[s]"}, {}])
+def test_single_append_cf(refs, mapper, dtype):
+    mzz = MultiZarrToZarr(
+        [refs["cfstdtime1"], refs["cfstdtime2"]],
+        remote_protocol="memory",
+        concat_dims=["time"],
+        coo_map=mapper,
+        coo_dtypes=dtype,
+    )
+    out = mzz.translate()
+    mzz = MultiZarrToZarr.append(
+        [refs["cfstdtime3"]],
+        out,
+        remote_protocol="memory",
+        concat_dims=["time"],
+        coo_map=mapper,
+        coo_dtypes=dtype,
+    )
+    out = mzz.translate()
+    z = xr.open_dataset(
+        "reference://",
+        backend_kwargs={
+            "storage_options": {"fo": out, "remote_protocol": "memory"},
+            "consolidated": False,
+        },
+        engine="zarr",
+    )
+    assert z.data.shape == (3, 10, 10)
+    assert out["refs"]["data/0.0.0"] == ["memory:///cfstdtime1.zarr/data/0.0.0"]
+    assert out["refs"]["data/1.0.0"] == ["memory:///cfstdtime2.zarr/data/0.0.0"]
+    assert out["refs"]["data/2.0.0"] == ["memory:///cfstdtime3.zarr/data/0.0.0"]
+    np.testing.assert_equal(
+        z.time.values,
+        np.array(
+            [
+                "1970-01-01T00:00:01.000000000",
+                "1970-01-01T00:00:02.000000000",
+                "1970-01-01T00:00:03.000000000",
+            ],
+            dtype="datetime64[ns]",
+        ),
+    )
+
+
 def test_single_append_parquet(refs):
     from fsspec.implementations.reference import LazyReferenceMapper
 
diff --git a/kerchunk/xarray_backend.py b/kerchunk/xarray_backend.py
index badf2907..ca377f6d 100644
--- a/kerchunk/xarray_backend.py
+++ b/kerchunk/xarray_backend.py
@@ -6,21 +6,14 @@
 
 class KerchunkBackend(BackendEntrypoint):
     def open_dataset(
-        self,
-        filename_or_obj,
-        *,
-        drop_variables=None,
-        storage_options=None,
-        open_dataset_options=None
+        self, filename_or_obj, *, storage_options=None, open_dataset_options=None, **kw
     ):
-
+        open_dataset_options = (open_dataset_options or {}) | kw
         ref_ds = open_reference_dataset(
             filename_or_obj,
             storage_options=storage_options,
             open_dataset_options=open_dataset_options,
         )
-        if drop_variables is not None:
-            ref_ds = ref_ds.drop_vars(drop_variables)
         return ref_ds
 
     open_dataset_parameters = [