Fix timezone handling in datetime_from_np_time

Closes #176

Author: mx-moth

continuous-integration/requirements-3.11.txt

@@ -51,6 +51,8 @@ flake8==7.3.0
     # via emsarray (pyproject.toml)
 fonttools==4.59.1
     # via matplotlib
+freezegun==1.5.5
+    # via emsarray (pyproject.toml)
 fsspec==2025.7.0
     # via dask
 geojson==3.2.0
@@ -188,6 +190,7 @@ pytest-mpl==0.17.0
     # via emsarray (pyproject.toml)
 python-dateutil==2.9.0.post0
     # via
+    #   freezegun
     #   matplotlib
     #   pandas
 pytz==2025.2

continuous-integration/requirements-3.12.txt

@@ -51,6 +51,8 @@ flake8==7.3.0
     # via emsarray (pyproject.toml)
 fonttools==4.59.1
     # via matplotlib
+freezegun==1.5.5
+    # via emsarray (pyproject.toml)
 fsspec==2025.7.0
     # via dask
 geojson==3.2.0
@@ -186,6 +188,7 @@ pytest-mpl==0.17.0
     # via emsarray (pyproject.toml)
 python-dateutil==2.9.0.post0
     # via
+    #   freezegun
     #   matplotlib
     #   pandas
 pytz==2025.2

continuous-integration/requirements-3.13.txt

@@ -51,6 +51,8 @@ flake8==7.3.0
     # via emsarray (pyproject.toml)
 fonttools==4.59.1
     # via matplotlib
+freezegun==1.5.5
+    # via emsarray (pyproject.toml)
 fsspec==2025.7.0
     # via dask
 geojson==3.2.0
@@ -186,6 +188,7 @@ pytest-mpl==0.17.0
     # via emsarray (pyproject.toml)
 python-dateutil==2.9.0.post0
     # via
+    #   freezegun
     #   matplotlib
     #   pandas
 pytz==2025.2

continuous-integration/requirements-minimum.txt

@@ -194,6 +194,6 @@ zipp~=3.20.2
 # emsarray
 # pytz
 # tzdata
-pytz
 tzdata
 certifi
+pytz

docs/releases/development.rst

@@ -9,3 +9,6 @@ Next release (in development)
   :ref:`how to set the clim parameter in plots <example-plot-with-clim>`
   (:pr:`179`).
 * Bumped pinned dependencies (:pr:`183`).
+* Fixed :func:`emsarray.utils.datetime_from_np_time`
+  when the system timezone is not UTC and a specific timezone is requested
+  (:issue:`176`, :pr:`183`).

pyproject.toml

@@ -64,6 +64,7 @@ testing = [
 	"types-pytz",
 	"flake8",
 	"isort",
+	"freezegun",
 ]
 
 [project.scripts]

src/emsarray/utils.py

@@ -695,15 +695,17 @@ def datetime_from_np_time(
         The timezone that the numpy datetime is in.
         Defaults to UTC, as xarray will convert all time variables to UTC when
         opening files.
+        The returned Python datetime will be in this timezone.
 
     Returns
     =======
     datetime.datetime
         A timezone aware Python datetime.datetime instance.
     """
-    epoc = numpy.datetime64('1970-01-01')
-    timestamp = (np_time - epoc).astype('timedelta64[ns]')
-    return datetime.datetime.fromtimestamp(timestamp.astype(float) / 1e9, tz=tz)
+    np_epoc = numpy.datetime64('1970-01-01')
+    ns_since_epoc = (np_time - np_epoc).astype('timedelta64[ns]')
+    py_epoc = datetime.datetime(1970, 1, 1, tzinfo=tz)
+    return py_epoc + datetime.timedelta(seconds=ns_since_epoc.astype(float) / 1e9)
 
 
 class RequiresExtraException(Exception):

tests/test_utils.py

@@ -3,6 +3,7 @@
 import pathlib
 from importlib.metadata import version
 
+import freezegun
 import netCDF4
 import numpy
 import numpy.testing
@@ -569,3 +570,25 @@ def test_wind_dimension_renamed():
     )
     wound = utils.wind_dimension(data_array, ['y', 'x'], [5, 4], linear_dimension='ix')
     xarray.testing.assert_equal(wound, expected)
+
+
+@pytest.mark.parametrize('tz_offset', [0, 10, -4])
+def test_datetime_from_np_time(tz_offset: int):
+    # Change the system timezone to `tz_offset`
+    with freezegun.freeze_time(tz_offset=tz_offset):
+        np_time = numpy.datetime64('2025-08-18T12:05:00.123456')
+
+        # Test that converting works using the UTC default timezone,
+        # regardless of system timezone
+        py_time_utc = utils.datetime_from_np_time(np_time)
+        assert py_time_utc == datetime.datetime(2025, 8, 18, 12, 5, 0, 123456, tzinfo=datetime.UTC)
+
+        # Test that converting works when interpreted in the system timezone.
+        py_tz_system = datetime.timezone(datetime.timedelta(hours=tz_offset))
+        py_time_local = utils.datetime_from_np_time(np_time, tz=py_tz_system)
+        assert py_time_local == datetime.datetime(2025, 8, 18, 12, 5, 0, 123456, tzinfo=py_tz_system)
+
+        # Test that converting works when using some other arbitrary timezone.
+        py_tz_eucla = datetime.timezone(datetime.timedelta(hours=8, minutes=45))
+        py_time_eucla = utils.datetime_from_np_time(np_time, tz=py_tz_eucla)
+        assert py_time_eucla == datetime.datetime(2025, 8, 18, 12, 5, 0, 123456, tzinfo=py_tz_eucla)