Decode time_dim based on monthlyVerificationDate

cfgrib/cfmessage.py

@@ -171,6 +171,10 @@ def build_valid_time(time, step):
         functools.partial(from_grib_date_time, date_key="indexingDate", time_key="indexingTime"),
         functools.partial(to_grib_date_time, date_key="indexingDate", time_key="indexingTime"),
     ),
+    "valid_month": (
+        functools.partial(from_grib_date_time, date_key="monthlyVerificationDate", time_key="validityTime"),
+        functools.partial(to_grib_date_time, date_key="monthlyVerificationDate", time_key="validityTime"),
+    ),
 }  # type: messages.ComputedKeysType
 
 

cfgrib/dataset.py

@@ -160,6 +160,7 @@
     "verifying_time",
     "forecastMonth",
     "indexing_time",
+    "valid_month",
 ]
 SPECTRA_KEYS = ["directionNumber", "frequencyNumber"]
 
@@ -250,6 +251,12 @@
         "standard_name": "time",
         "long_name": "time",
     },
+    "valid_month": {
+        "units": "seconds since 1970-01-01T00:00:00",
+        "calendar": "proleptic_gregorian",
+        "standard_name": "time",
+        "long_name": "time",
+    },
     "verifying_time": {
         "units": "seconds since 1970-01-01T00:00:00",
         "calendar": "proleptic_gregorian",
@@ -457,10 +464,7 @@ def encode_cf_first(data_var_attrs, encode_cf=("parameter", "time"), time_dims=(
         if "GRIB_units" in data_var_attrs:
             data_var_attrs["units"] = data_var_attrs["GRIB_units"]
     if "time" in encode_cf:
-        if set(time_dims).issubset(ALL_REF_TIME_KEYS):
-            coords_map.extend(time_dims)
-        else:
-            raise ValueError("time_dims %r not a subset of %r" % (time_dims, ALL_REF_TIME_KEYS))
+        coords_map.extend(time_dims)
     else:
         coords_map.extend(DATA_TIME_KEYS)
     coords_map.extend(VERTICAL_KEYS)
@@ -498,8 +502,9 @@ def build_variable_components(
     first = index.first()
     extra_attrs = read_data_var_attrs(first, extra_keys)
     data_var_attrs.update(**extra_attrs)
-    coords_map = encode_cf_first(data_var_attrs, encode_cf, time_dims)
-
+    coords_map = encode_cf_first(
+        data_var_attrs, encode_cf, time_dims,
+    )
     coord_name_key_map = {}
     coord_vars = {}
     for coord_key in coords_map:
@@ -667,6 +672,14 @@ def build_dataset_components(
     variables = {}  # type: T.Dict[str, Variable]
     filter_by_keys = index.filter_by_keys
 
+    # Warn about time_dims here to prevent repeasted messages in build_variable_components
+    if errors != "ignore" and not set(time_dims).issubset(ALL_REF_TIME_KEYS):
+        log.warning(
+            "Not all time_dimensions are recognised, those which are not in the following list will not "
+            " be decoded as datetime objects:\n"
+            f"{ALL_REF_TIME_KEYS}"
+        )
+
     for param_id in index.get("paramId", []):
         var_index = index.subindex(paramId=param_id)
         try:

tests/test_50_sample_data.py

@@ -120,3 +120,12 @@ def test_dataset_missing_field_values() -> None:
     t2 = res.variables["t2m"]
     assert np.isclose(np.nanmean(t2[0, :, :]), 268.375)
     assert np.isclose(np.nanmean(t2[1, :, :]), 270.716)
+
+
+def test_valid_month_time_dim() -> None:
+
+    test_file = os.path.join(SAMPLE_DATA_FOLDER, "cams-egg4-monthly.grib")
+    ds = xr.open_dataset(test_file, time_dims=["valid_month"])
+
+    assert "valid_month" in ds.dims
+