feat: add "complete" timezone support

at least for now, this is all we need i think..  if nothing
configured, will fallback to default system local timezone.  or can
configure the default, or alternate(s) as needed.

some day when we drop support for python 3.8, can also remove the
python-dateutil dependency..

tests/test_app.py

@@ -16,9 +16,10 @@ from mako.template import Template
 
 import wuttjamaican.enum
 from wuttjamaican import app as mod
+from wuttjamaican.exc import ConfigurationError
 from wuttjamaican.progress import ProgressBase
 from wuttjamaican.conf import WuttaConfig
-from wuttjamaican.util import UNSPECIFIED
+from wuttjamaican.util import UNSPECIFIED, get_timezone_by_name
 from wuttjamaican.testing import FileTestCase, ConfigTestCase
 from wuttjamaican.batch import BatchHandler
 
@@ -426,6 +427,80 @@ app_title = WuttaTest
         name = self.app.make_full_name("Fred", "", "Flintstone", "")
         self.assertEqual(name, "Fred Flintstone")
 
+    def test_get_timezone(self):
+        # cache is empty at first
+        self.assertEqual(self.app.timezones, {})
+
+        # fetch default system local timezone
+        # nb. actual value depends on machine where tests run
+        system = self.app.get_timezone()
+        self.assertIsInstance(system, datetime.tzinfo)
+        # cache no longer empty
+        self.assertEqual(len(self.app.timezones), 1)
+        self.assertIn("default", self.app.timezones)
+        self.assertIs(self.app.timezones["default"], system)
+
+        # fetch configured default
+        self.app.timezones.clear()  # clear cache
+        self.config.setdefault("wuttatest.timezone.default", "Africa/Addis_Ababa")
+        default = self.app.get_timezone()
+        self.assertIsInstance(default, datetime.tzinfo)
+        dt = datetime.datetime(2025, 12, 16, 22, 0, tzinfo=default)
+        self.assertEqual(default.utcoffset(dt), datetime.timedelta(hours=3))
+        # cache no longer empty
+        self.assertEqual(len(self.app.timezones), 1)
+        self.assertIn("default", self.app.timezones)
+        self.assertIs(self.app.timezones["default"], default)
+        # fetching again gives cached instance
+        self.assertIs(self.app.get_timezone(), default)
+
+        # fetch configured alternate
+        self.config.setdefault("wuttatest.timezone.foo", "America/New_York")
+        foo = self.app.get_timezone("foo")
+        self.assertIsInstance(foo, datetime.tzinfo)
+        self.assertIn("foo", self.app.timezones)
+        self.assertIs(self.app.timezones["foo"], foo)
+
+        # error if alternate not configured
+        self.assertRaises(ConfigurationError, self.app.get_timezone, "bar")
+        self.assertNotIn("bar", self.app.timezones)
+
+    def test_get_timezone_name(self):
+        # cache is empty at first
+        self.assertEqual(self.app.timezones, {})
+
+        # fetch default system local timezone
+        # nb. actual value depends on machine where tests run
+        system = self.app.get_timezone_name()
+        self.assertIsInstance(system, str)
+        self.assertLess(0, len(system))
+        # cache no longer empty
+        self.assertEqual(len(self.app.timezones), 1)
+        self.assertIn("default", self.app.timezones)
+
+        # fetch configured default
+        self.app.timezones.clear()  # clear cache
+        self.config.setdefault("wuttatest.timezone.default", "Africa/Addis_Ababa")
+        default = self.app.get_timezone_name()
+        # nb. this check won't work for python 3.8
+        if sys.version_info >= (3, 9):
+            self.assertEqual(default, "Africa/Addis_Ababa")
+        # cache no longer empty
+        self.assertEqual(len(self.app.timezones), 1)
+        self.assertIn("default", self.app.timezones)
+
+        # fetch configured alternate
+        self.config.setdefault("wuttatest.timezone.foo", "America/New_York")
+        foo = self.app.get_timezone_name("foo")
+        # nb. this check won't work for python 3.8
+        if sys.version_info >= (3, 9):
+            self.assertEqual(foo, "America/New_York")
+        self.assertIn("foo", self.app.timezones)
+
+        # error if alternate not configured
+        self.assertRaises(ConfigurationError, self.app.get_timezone_name, "bar")
+        self.assertNotIn("bar", self.app.timezones)
+
     def test_localtime(self):
         dt = self.app.localtime()
         self.assertIsInstance(dt, datetime.datetime)
@@ -516,15 +591,38 @@ app_title = WuttaTest
         self.assertEqual(self.app.render_date(dt), "2024-12-11")
 
     def test_render_datetime(self):
-        self.assertEqual(self.app.render_datetime(None), "")
+        tzlocal = get_timezone_by_name("America/Los_Angeles")
+        with patch.object(self.app, "get_timezone", return_value=tzlocal):
 
-        dt = datetime.datetime(2024, 12, 11, 8, 30, tzinfo=datetime.timezone.utc)
-        self.assertEqual(self.app.render_datetime(dt), "2024-12-11 08:30+0000")
+            # null value
+            self.assertEqual(self.app.render_datetime(None), "")
 
-        dt = datetime.datetime(2024, 12, 11, 8, 30)
-        text = self.app.render_datetime(dt)
-        # TODO: should override local timezone for more complete test
-        self.assertTrue(text.startswith("2024-12-"))
+            # naive UTC
+            dt = datetime.datetime(2024, 12, 17, 1, 12)
+            self.assertEqual(
+                self.app.render_datetime(dt, local=True), "2024-12-16 17:12-0800"
+            )
+            self.assertEqual(
+                self.app.render_datetime(dt, local=False), "2024-12-17 01:12"
+            )
+
+            # aware UTC
+            dt = datetime.datetime(2024, 12, 17, 1, 12, tzinfo=datetime.timezone.utc)
+            self.assertEqual(
+                self.app.render_datetime(dt, local=True), "2024-12-16 17:12-0800"
+            )
+            self.assertEqual(
+                self.app.render_datetime(dt, local=False), "2024-12-17 01:12+0000"
+            )
+
+            # aware local
+            dt = datetime.datetime(2024, 12, 16, 19, 12, tzinfo=tzlocal)
+            self.assertEqual(
+                self.app.render_datetime(dt, local=True), "2024-12-16 19:12-0800"
+            )
+            self.assertEqual(
+                self.app.render_datetime(dt, local=False), "2024-12-16 19:12-0800"
+            )
 
     def test_render_error(self):