feat(cli): add flexible positional day argument

- Add parse_date_arg() to utils.py supporting YYYY-MM-DD, MM-DD, and
  DD-MM formats with either - or / as separator
- Add AmbiguousDateError for two-part dates valid as both MM-DD and DD-MM
- Replace --day flag with a positional optional argument (defaults to today)
- Remove old _parse_date() helper from cli.py

tests/test_utils.py

@@ -3,9 +3,11 @@ from datetime import date
 import pytest
 
 from timesheets.utils import (
+    AmbiguousDateError,
     decimal_to_hhmm,
     duration_from_start_end,
     format_date,
+    parse_date_arg,
     parse_duration,
     strip_markdown_link,
 )
@@ -98,3 +100,75 @@ class TestFormatDate:
 
     def test_single_digit_day_month(self):
         assert format_date(date(2026, 1, 5)) == "05/01/26"
+
+
+class TestParseDateArg:
+    # Fix the year used for two-part tests to avoid test brittleness
+    THIS_YEAR = 2026
+
+    def _parse(self, value: str) -> date:
+        # Patch date.today so two-part tests are year-stable
+        from unittest.mock import patch
+
+        with patch("timesheets.utils.date") as mock_date:
+            mock_date.today.return_value = date(self.THIS_YEAR, 6, 1)
+            mock_date.side_effect = lambda *a, **kw: date(*a, **kw)
+            return parse_date_arg(value)
+
+    # --- YYYY-MM-DD ---
+    def test_full_dash(self):
+        assert parse_date_arg("2026-05-22") == date(2026, 5, 22)
+
+    def test_full_slash(self):
+        assert parse_date_arg("2026/05/22") == date(2026, 5, 22)
+
+    def test_full_invalid_day(self):
+        with pytest.raises(ValueError):
+            parse_date_arg("2026-02-30")
+
+    # --- two-part unambiguous: only valid as MM-DD ---
+    def test_mm_dd_unambiguous(self):
+        # 01-22: valid as MM=1, DD=22; invalid as MM=22, DD=1
+        result = self._parse("01-22")
+        assert result == date(self.THIS_YEAR, 1, 22)
+
+    # --- two-part unambiguous: only valid as DD-MM ---
+    def test_dd_mm_unambiguous(self):
+        # 22-01: invalid as MM=22, DD=1 is actually valid... use 22-13
+        # 22-13: valid as DD=22, MM=13 is invalid; DD=13 MM=22 is invalid too
+        # Use 30-11: valid as DD=30,MM=11; invalid as MM=30,DD=11
+        result = self._parse("30-11")
+        assert result == date(self.THIS_YEAR, 11, 30)
+
+    def test_slash_separator(self):
+        result = self._parse("01/22")
+        assert result == date(self.THIS_YEAR, 1, 22)
+
+    # --- ambiguous ---
+    def test_ambiguous_raises(self):
+        # 05-06: valid as MM=5,DD=6 and as DD=5,MM=6
+        with pytest.raises(AmbiguousDateError, match="ambiguous"):
+            self._parse("05-06")
+
+    def test_ambiguous_slash_raises(self):
+        with pytest.raises(AmbiguousDateError):
+            self._parse("05/06")
+
+    # --- identical interpretations are not ambiguous ---
+    def test_same_day_month_not_ambiguous(self):
+        # 05-05: MM=5,DD=5 and DD=5,MM=5 are the same date
+        result = self._parse("05-05")
+        assert result == date(self.THIS_YEAR, 5, 5)
+
+    # --- fully invalid ---
+    def test_invalid_both_parts(self):
+        # 13-32 is invalid as both MM-DD and DD-MM
+        with pytest.raises(ValueError):
+            self._parse("13-32")
+
+    def test_unrecognised_format(self):
+        with pytest.raises(ValueError, match="Unrecognised"):
+            parse_date_arg("not-a-date")
+
+    def test_whitespace_stripped(self):
+        assert parse_date_arg("  2026-05-22  ") == date(2026, 5, 22)