timestamp: Use localized formatting in format_datetime_to_string.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

pyproject.toml

@@ -210,6 +210,9 @@ prod = [
 
   # Better compression than zlib
   "zstd",
+
+  # Internationalization
+  "pyicu",
 ]
 docs = [
   # Needed to build RTD docs
@@ -406,6 +409,7 @@ module = [
     "fakeldap.*",
     "firebase_admin.*",
     "gitlint.*",
+    "icu.*", # https://gitlab.pyicu.org/main/pyicu/-/issues/156
     "integrations.*",
     "jsonref.*",
     "ldap.*", # https://github.com/python-ldap/python-ldap/issues/368

scripts/lib/setup_venv.py

@@ -24,6 +24,7 @@ VENV_DEPENDENCIES = [
     "libsasl2-dev",  # For building python-ldap from source
     "libvips",  # For thumbnailing
     "libvips-tools",
+    "libicu-dev",  # For building pyicu
 ]
 
 COMMON_YUM_VENV_DEPENDENCIES = [

uv.lock

@@ -3505,6 +3505,12 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/c7/21/705964c7812476f378728bdf590ca4b771ec72385c533964653c68e86bdc/pygments-2.19.2-py3-none-any.whl", hash = "sha256:86540386c03d588bb81d44bc3928634ff26449851e99741617ecb9037ee5ec0b", size = 1225217, upload-time = "2025-06-21T13:39:07.939Z" },
 ]
 
+[[package]]
+name = "pyicu"
+version = "2.15.3"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/88/b0/c8b61bac55424e2ff80e20d7251c3f002baff3c07c34cee3849e3505d8f5/pyicu-2.15.3.tar.gz", hash = "sha256:f32e78e1cb64d0aeb14f027e037a8944861d3114548818a6adf0081ef51aefc3", size = 267569, upload-time = "2025-09-15T20:58:50.936Z" }
+
 [[package]]
 name = "pyinotify"
 version = "0.9.6"
@@ -5830,6 +5836,7 @@ dev = [
     { name = "pydantic" },
     { name = "pydantic-partials" },
     { name = "pygments" },
+    { name = "pyicu" },
     { name = "pyinotify" },
     { name = "pyjwt" },
     { name = "pymongo" },
@@ -5954,6 +5961,7 @@ prod = [
     { name = "pydantic" },
     { name = "pydantic-partials" },
     { name = "pygments" },
+    { name = "pyicu" },
     { name = "pyjwt" },
     { name = "pymongo" },
     { name = "pynacl" },
@@ -6057,6 +6065,7 @@ dev = [
     { name = "pydantic" },
     { name = "pydantic-partials" },
     { name = "pygments" },
+    { name = "pyicu" },
     { name = "pyinotify" },
     { name = "pyjwt" },
     { name = "pymongo" },
@@ -6182,6 +6191,7 @@ prod = [
     { name = "pydantic" },
     { name = "pydantic-partials" },
     { name = "pygments" },
+    { name = "pyicu" },
     { name = "pyjwt" },
     { name = "pymongo" },
     { name = "pynacl" },

version.py

@@ -49,4 +49,4 @@ API_FEATURE_LEVEL = 427
 #   historical commits sharing the same major version, in which case a
 #   minor version bump suffices.
 
-PROVISION_VERSION = (351, 0)  # bumped 2025-10-01 to remove emoji-datasource-google-blob
+PROVISION_VERSION = (351, 1)  # bumped 2025-10-02 to add PyICU

zerver/lib/timestamp.py

@@ -1,4 +1,8 @@
 from datetime import datetime, timedelta, timezone
+from functools import cache
+
+import icu
+from django.utils.translation import get_language
 
 
 class TimeZoneNotUTCError(Exception):
@@ -49,9 +53,35 @@ def datetime_to_timestamp(dt: datetime) -> int:
     return int(dt.timestamp())
 
 
+@cache
+def get_date_time_pattern_generator(language: str) -> icu.DateTimePatternGenerator:
+    return icu.DateTimePatternGenerator.createInstance(icu.Locale(language))
+
+
+@cache
+def get_icu_time_zone(time_zone: str) -> icu.TimeZone:
+    return icu.TimeZone.createTimeZone(time_zone)
+
+
+@cache
+def get_date_time_format(language: str, use_twenty_four_hour_time: bool) -> icu.SimpleDateFormat:
+    skeleton = f"yMMMEd{'H' if use_twenty_four_hour_time else 'h'}mz"
+    pattern = get_date_time_pattern_generator(language).getBestPattern(skeleton)
+    return icu.SimpleDateFormat(pattern, icu.Locale(language))
+
+
 def format_datetime_to_string(dt: datetime, use_twenty_four_hour_time: bool) -> str:
-    if use_twenty_four_hour_time:
-        hhmm_string = dt.strftime("%H:%M")
-    else:
-        hhmm_string = dt.strftime("%I:%M %p")
-    return dt.strftime(f"%A, %B %d, %Y at {hhmm_string} %Z")
+    assert dt.tzinfo is not None
+    time_zone = getattr(dt.tzinfo, "key", None)
+    if time_zone is None:
+        offset = dt.tzinfo.utcoffset(dt)
+        assert offset is not None
+        sign = "-" if offset < timedelta(0) else "+"
+        hours, rest = divmod(abs(offset), timedelta(hours=1))
+        minutes, rest = divmod(rest, timedelta(minutes=1))
+        assert rest == timedelta(0)
+        time_zone = f"GMT{sign}{hours:02}:{minutes:02}"
+    language = get_language()
+    calendar = icu.Calendar.createInstance(get_icu_time_zone(time_zone), icu.Locale(language))
+    calendar.setTime(dt)
+    return get_date_time_format(language, use_twenty_four_hour_time).format(calendar)

zerver/tests/test_message_notification_emails.py

@@ -1426,9 +1426,9 @@ class TestMessageNotificationEmails(ZulipTestCase):
         self.assertEqual(
             "> Meeting at <time:2025-09-30T09:30:00-07:00>\n\n--\n\nReply", mail.outbox[0].body[:56]
         )
-        self.assertIn(
-            '<p>Meeting at <time datetime="2025-09-30T16:30:00Z">Tuesday, September 30, 2025 at 04:30 PM UTC</time></p>',
+        self.assertRegex(
             mail.outbox[0].alternatives[0][0],
+            r'<p>Meeting at <time datetime="2025-09-30T16:30:00Z">Tue, Sep 30, 2025, 4:30[ \u202f]PM UTC</time></p>',
         )
 
         # The timestamp is not formatted correctly.

zerver/tests/test_new_users.py

@@ -1,4 +1,3 @@
-import zoneinfo
 from collections.abc import Sequence
 from datetime import datetime, timedelta, timezone
 
@@ -14,7 +13,6 @@ from zerver.actions.streams import do_set_stream_property
 from zerver.actions.user_settings import do_change_user_setting
 from zerver.lib.initial_password import initial_password
 from zerver.lib.test_classes import ZulipTestCase
-from zerver.lib.timezone import canonicalize_timezone
 from zerver.models import Message, Recipient, Stream, UserProfile
 from zerver.models.realms import get_realm
 from zerver.models.recipients import get_direct_message_group_user_ids
@@ -53,9 +51,7 @@ class SendLoginEmailTest(ZulipTestCase):
             firefox_windows = (
                 "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0"
             )
-            user_tz = zoneinfo.ZoneInfo(canonicalize_timezone(user.timezone))
             mock_time = datetime(year=2018, month=1, day=1, tzinfo=timezone.utc)
-            reference_time = mock_time.astimezone(user_tz).strftime("%A, %B %d, %Y at %I:%M %p %Z")
             with time_machine.travel(mock_time, tick=False):
                 self.client_post(
                     "/accounts/login/", info=login_info, HTTP_USER_AGENT=firefox_windows
@@ -66,7 +62,7 @@ class SendLoginEmailTest(ZulipTestCase):
             subject = "New login from Firefox on Windows"
             self.assertEqual(mail.outbox[0].subject, subject)
             # local time is correct and in email's body
-            self.assertIn(reference_time, mail.outbox[0].body)
+            self.assertRegex(str(mail.outbox[0].body), r"Sun, Dec 31, 2017, 4:00[ \u202f]PM PST")
 
             # Try again with the 24h time format setting enabled for this user
             self.logout()  # We just logged in, we'd be redirected without this
@@ -77,7 +73,7 @@ class SendLoginEmailTest(ZulipTestCase):
                     "/accounts/login/", info=login_info, HTTP_USER_AGENT=firefox_windows
                 )
 
-            reference_time = mock_time.astimezone(user_tz).strftime("%A, %B %d, %Y at %H:%M %Z")
+            reference_time = "Sun, Dec 31, 2017, 16:00 PST"
             self.assertIn(reference_time, mail.outbox[1].body)
 
     def test_dont_send_login_emails_if_send_login_emails_is_false(self) -> None:

zerver/tests/test_timestamp.py

@@ -1,6 +1,8 @@
-from datetime import timedelta, timezone
+from datetime import datetime, timedelta, timezone
+from zoneinfo import ZoneInfo
 
 from dateutil import parser
+from django.utils.translation import override as override_language
 
 from zerver.lib.test_classes import ZulipTestCase
 from zerver.lib.timestamp import (
@@ -10,6 +12,7 @@ from zerver.lib.timestamp import (
     datetime_to_timestamp,
     floor_to_day,
     floor_to_hour,
+    format_datetime_to_string,
     timestamp_to_datetime,
 )
 
@@ -45,3 +48,21 @@ class TestTimestamp(ZulipTestCase):
         for function in [floor_to_hour, floor_to_day, ceiling_to_hour, ceiling_to_hour]:
             with self.assertRaises(TimeZoneNotUTCError):
                 function(non_utc_datetime)
+
+    def test_format_datetime_to_string(self) -> None:
+        dt = datetime(2001, 2, 3, 4, 5, 6, tzinfo=timezone.utc)
+        self.assertEqual(format_datetime_to_string(dt, True), "Sat, Feb 3, 2001, 04:05 GMT")
+        dt = datetime(2001, 2, 3, 4, 5, 6, tzinfo=timezone(timedelta(hours=7, minutes=8)))
+        self.assertEqual(format_datetime_to_string(dt, True), "Sat, Feb 3, 2001, 04:05 GMT+7:08")
+        dt = datetime(2001, 2, 3, 4, 5, 6, tzinfo=timezone(-timedelta(hours=7, minutes=8)))
+        self.assertEqual(format_datetime_to_string(dt, True), "Sat, Feb 3, 2001, 04:05 GMT-7:08")
+        dt = datetime(2001, 2, 3, 4, 5, 6, tzinfo=ZoneInfo("America/Los_Angeles"))
+        self.assertEqual(format_datetime_to_string(dt, True), "Sat, Feb 3, 2001, 04:05 PST")
+        self.assertRegex(
+            format_datetime_to_string(dt, False), r"^Sat, Feb 3, 2001, 4:05[ \u202f]AM PST$"
+        )
+        with override_language("ja-JP"):
+            self.assertEqual(format_datetime_to_string(dt, True), "2001年2月3日(土) 4:05 GMT-8")
+            self.assertEqual(
+                format_datetime_to_string(dt, False), "2001年2月3日(土) 午前4:05 GMT-8"
+            )