feat: inject current date/time context into chat system prompt

Without this Claude has no grounding for "heute"/"morgen"/"naechste
Woche" and only understands dates the user states explicitly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V57jSQPqwkGG8BuAXg59X5

Claude outputs/main.py

@@ -409,6 +409,25 @@ def _as_calendar_local(dt: datetime) -> datetime:
     return dt
 
 
+_WEEKDAYS_DE = ["Montag", "Dienstag", "Mittwoch", "Donnerstag", "Freitag", "Samstag", "Sonntag"]
+_MONTHS_DE = [
+    "Januar", "Februar", "Maerz", "April", "Mai", "Juni",
+    "Juli", "August", "September", "Oktober", "November", "Dezember",
+]
+
+
+def _current_datetime_context(now: datetime = None) -> str:
+    """Chat has no other grounding for 'heute'/'morgen'/'naechste Woche' -
+    without this, Claude only knows dates the user states explicitly."""
+    now = now or datetime.now(CALENDAR_TIMEZONE)
+    weekday = _WEEKDAYS_DE[now.weekday()]
+    month = _MONTHS_DE[now.month - 1]
+    return (
+        f"Aktuelles Datum und Uhrzeit: {weekday}, {now.day}. {month} {now.year}, "
+        f"{now.strftime('%H:%M')} Uhr ({now.strftime('%d.%m.%Y')})."
+    )
+
+
 def _create_event_sync(summary: str, start: str, end: str, description: str = "", calendar: str = DEFAULT_CALENDAR) -> dict:
     cal = _caldav_calendar(calendar)
     cal.save_event(
@@ -1026,7 +1045,7 @@ async def run_chat_completion(claude_messages: list, conversation_id: int):
     memory_context = await build_memory_context(latest_user_message, conversation_id)
 
     system_prompt = (
-        f"{CLAUDE_SYSTEM_PROMPT}\n\n{CALENDAR_ASSISTANT_INSTRUCTIONS}\n\n"
+        f"{CLAUDE_SYSTEM_PROMPT}\n\n{_current_datetime_context()}\n\n{CALENDAR_ASSISTANT_INSTRUCTIONS}\n\n"
         f"{MEMORY_ASSISTANT_INSTRUCTIONS}\n\n{ORDER_ASSISTANT_INSTRUCTIONS}"
     )
     if memory_context:

Claude outputs/tests/test_chat_tools.py

@@ -1,5 +1,6 @@
 import os
 import sys
+from datetime import datetime
 from unittest.mock import AsyncMock, MagicMock, patch
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
@@ -143,3 +144,32 @@ async def test_run_chat_completion_lists_recent_emails():
     assert text == "Du hast 5 neue Mails."
     assert output_tokens == 6 + 8
     assert total_tokens == 15 + 6 + 20 + 8
+
+
+def test_current_datetime_context_formats_german_weekday_and_month():
+    fixed = datetime(2026, 9, 13, 21, 15, tzinfo=main.CALENDAR_TIMEZONE)
+
+    context = main._current_datetime_context(fixed)
+
+    assert context == (
+        "Aktuelles Datum und Uhrzeit: Sonntag, 13. September 2026, 21:15 Uhr (13.09.2026)."
+    )
+
+
+@pytest.mark.asyncio
+async def test_run_chat_completion_includes_current_datetime_in_system_prompt():
+    completion = MagicMock()
+    completion.stop_reason = "end_turn"
+    completion.content = [_text_block("Hallo!")]
+    completion.usage = _usage(10, 5)
+
+    main.claude_client = MagicMock()
+    main.claude_client.messages.create.return_value = completion
+
+    with patch.object(main, "build_memory_context", new=AsyncMock(return_value="")), patch.object(
+        main, "_current_datetime_context", return_value="Aktuelles Datum und Uhrzeit: TESTMARKER"
+    ):
+        await main.run_chat_completion([{"role": "user", "content": "Hi"}], conversation_id=1)
+
+    _, kwargs = main.claude_client.messages.create.call_args
+    assert "TESTMARKER" in kwargs["system"]