diff --git a/planet/api/filters.py b/planet/api/filters.py
index 60abaab..baa07d9 100644
--- a/planet/api/filters.py
+++ b/planet/api/filters.py
@@ -100,8 +100,10 @@ def not_filter(*predicates):
 def date_range(field_name, **kwargs):
     '''Build a DateRangeFilter.
 
-    Predicate arguments accept a value str that in ISO-8601 format or a value
-    that has a `isoformat` callable that returns an ISO-8601 str.
+    Predicate arguments accept a str that is ISO-8601 format or a value
+    that has an `isoformat` callable that returns an ISO-8601 compliant str.
+
+    If no timezone is provided, UTC is assumed for RFC 3339 compatability.
 
     :raises: ValueError if predicate value does not parse
 
@@ -116,7 +118,12 @@ def date_range(field_name, **kwargs):
             dt = strp_lenient(str(v))
             if dt is None:
                 raise ValueError("unable to use provided time: " + str(v))
-        kwargs[k] = dt.isoformat() + 'Z'
+        iso_date = dt.isoformat()
+        if not dt.tzinfo:
+            # assume UTC for datetimes without an explicit timezone
+            # necessary for RFC 3339 vs ISO 8601 compatability
+            iso_date += 'Z'
+        kwargs[k] = iso_date
     return _filter('DateRangeFilter', config=kwargs, field_name=field_name)
 
 
diff --git a/tox.ini b/tox.ini
index a410f23..ab3b9d2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,10 +4,10 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, py34, py35, py36, py37
+envlist = py27, py36, py37
 
 [testenv]
 deps = pytest
 commands = 
     pip install -e .[dev]
-    pytest
+    pytest {posargs}

Date range filter with timezone
Request with the `date_range` fails for `datetime.datetime` objects with the timezone specified
```
$ tz = pytz.timezone('UTC')
$ dt = datetime.datetime(2018,1,1, tzinfo=tz)
$ date_range('acquired', gte=dt)
>>> {'field_name': 'acquired', 'type': 'DateRangeFilter', 'config': {'gte': '2018-01-01T00:00:00+00:00Z'}}
```
It can be seen that `Z` is still appended to the end, which does not seem to be valid ISO8601 string.
diff --git a/tests/test_filters.py b/tests/test_filters.py
new file mode 100644
index 0000000..ccbcd7d
--- /dev/null
+++ b/tests/test_filters.py
@@ -0,0 +1,27 @@
+import pytest
+from pytz import timezone
+from datetime import datetime
+from planet.api import filters
+
+
+@pytest.mark.parametrize('dt, expected', [
+    (datetime(1900, 1, 1, tzinfo=timezone("US/Central")),
+     {'field_name': 'acquired',
+      'type': 'DateRangeFilter',
+      'config': {'gte': '1900-01-01T00:00:00-05:51'}}),
+
+    (datetime(1999, 12, 31),
+     {'field_name': 'acquired',
+      'type': 'DateRangeFilter',
+      'config': {'gte': '1999-12-31T00:00:00Z'}}),
+
+    ("2018-01-01",
+     {'field_name': 'acquired',
+      'type': 'DateRangeFilter',
+      'config': {'gte': '2018-01-01T00:00:00Z'}}),
+])
+def test_date_range(dt, expected):
+    fieldname = "acquired"
+    arg = "gte"
+
+    assert filters.date_range(fieldname, **{arg: dt}) == expected
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 00b4405..53dafbc 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -148,7 +148,7 @@ def test_get_filename_from_url(url, expected):
 
 @pytest.mark.parametrize('content_type,check', [
     (None, lambda x: re.match(r'^planet-[a-z0-9]{8}$', x, re.I) is not None),
-    ('image/tiff', lambda x: x.endswith('.tif')),
+    ('image/tiff', lambda x: x.endswith(('.tif', '.tiff'))),
 ])
 def test_get_random_filename(content_type, check):
     assert check(utils.get_random_filename(content_type))

pip install -e .[dev]
parse_log_pytest
requirements.txt
mock pytest requests-mock flake8 setuptools pex pytest-cov sphinx wheel
3.7
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.13
Babel==2.14.0
certifi @ file:///croot/certifi_1671487769961/work/certifi
charset-normalizer==3.4.1
click==8.1.8
coverage==7.2.7
docutils==0.17.1
exceptiongroup==1.2.2
flake8==5.0.4
idna==3.10
imagesize==1.4.1
importlib-metadata==4.2.0
iniconfig==2.0.0
Jinja2==3.1.6
MarkupSafe==2.1.5
mccabe==0.7.0
mock==5.2.0
packaging==24.0
pex==2.33.7
-e git+https://github.com/planetlabs/planet-client-python.git@1c62ce7d416819951dddee0c22068fef6d40b027#egg=planet
pluggy==1.2.0
pycodestyle==2.9.1
pyflakes==2.5.0
Pygments==2.17.2
pytest==7.4.4
pytest-cov==4.1.0
pytz==2025.2
requests==2.31.0
requests-futures==1.0.2
requests-mock==1.12.1
snowballstemmer==2.2.0
Sphinx==4.3.2
sphinxcontrib-applehelp==1.0.2
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==2.0.0
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.5
tomli==2.0.1
typing_extensions==4.7.1
urllib3==2.0.7
zipp==3.15.0

name: planet-client-python
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2022.12.7=py37h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - pip=22.3.1=py37h06a4308_0
  - python=3.7.16=h7a1cb2a_0
  - readline=8.2=h5eee18b_0
  - setuptools=65.6.3=py37h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - wheel=0.38.4=py37h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.13
      - babel==2.14.0
      - charset-normalizer==3.4.1
      - click==8.1.8
      - coverage==7.2.7
      - docutils==0.17.1
      - exceptiongroup==1.2.2
      - flake8==5.0.4
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==4.2.0
      - iniconfig==2.0.0
      - jinja2==3.1.6
      - markupsafe==2.1.5
      - mccabe==0.7.0
      - mock==5.2.0
      - packaging==24.0
      - pex==2.33.7
      - pluggy==1.2.0
      - pycodestyle==2.9.1
      - pyflakes==2.5.0
      - pygments==2.17.2
      - pytest==7.4.4
      - pytest-cov==4.1.0
      - pytz==2025.2
      - requests==2.31.0
      - requests-futures==1.0.2
      - requests-mock==1.12.1
      - snowballstemmer==2.2.0
      - sphinx==4.3.2
      - sphinxcontrib-applehelp==1.0.2
      - sphinxcontrib-devhelp==1.0.2
      - sphinxcontrib-htmlhelp==2.0.0
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==1.0.3
      - sphinxcontrib-serializinghtml==1.1.5
      - tomli==2.0.1
      - typing-extensions==4.7.1
      - urllib3==2.0.7
      - zipp==3.15.0
prefix: /opt/conda/envs/planet-client-python

tests/test_filters.py::test_date_range[dt0-expected0]
tests/test_filters.py::test_date_range[dt1-expected1] tests/test_filters.py::test_date_range[2018-01-01-expected2] tests/test_utils.py::test_strp_lenient tests/test_utils.py::test_geometry_from_json tests/test_utils.py::test_probably_wkt tests/test_utils.py::test_probably_geojson tests/test_utils.py::test_check_status tests/test_utils.py::test_write_to_file tests/test_utils.py::test_get_filename_from_headers[headers0-open_california.tif] tests/test_utils.py::test_get_filename_from_headers[headers1-None] tests/test_utils.py::test_get_filename_from_headers[headers2-None] tests/test_utils.py::test_get_filename_from_url[https://planet.com/-None] tests/test_utils.py::test_get_filename_from_url[https://planet.com/path/to/-None] tests/test_utils.py::test_get_filename_from_url[https://planet.com/path/to/example.tif-example.tif] tests/test_utils.py::test_get_filename_from_url[https://planet.com/path/to/example.tif?foo=f6f1&bar=baz-example.tif] tests/test_utils.py::test_get_filename_from_url[https://planet.com/path/to/example.tif?foo=f6f1#quux-example.tif] tests/test_utils.py::test_get_random_filename[None-<lambda>] tests/test_utils.py::test_get_random_filename[image/tiff-<lambda>]
tox.ini planet/api/filters.py
tox.ini planet/api/filters.py