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

src/timesheets/utils.py

@@ -1,6 +1,73 @@
 import re
 from datetime import date
 
+# Separator is either - or /
+_SEP = r"[-/]"
+_DATE_PATTERNS = [
+    # YYYY-MM-DD or YYYY/MM/DD
+    re.compile(r"^(\d{4})" + _SEP + r"(\d{1,2})" + _SEP + r"(\d{1,2})$"),
+    # Two-part: first two digits, sep, last two digits
+    re.compile(r"^(\d{1,2})" + _SEP + r"(\d{1,2})$"),
+]
+
+
+class AmbiguousDateError(ValueError):
+    """Raised when a two-part date is valid as both MM-DD and DD-MM."""
+
+
+def _valid_date(year: int, month: int, day: int) -> date | None:
+    """Return a date if the triple is valid, else None."""
+    try:
+        return date(year, month, day)
+    except ValueError:
+        return None
+
+
+def parse_date_arg(value: str) -> date:
+    """
+    Parse a flexible date string into a date object.
+
+    Accepted formats (separator is - or /):
+        YYYY-MM-DD   unambiguous
+        MM-DD        month then day; year assumed to be current year
+        DD-MM        day then month; year assumed to be current year
+
+    If a two-part value is valid as both MM-DD and DD-MM, raises
+    AmbiguousDateError. If it is valid as neither, raises ValueError.
+    """
+    value = value.strip()
+    today = date.today()
+
+    # Try YYYY-MM-DD first
+    m = re.match(r"^(\d{4})[-/](\d{1,2})[-/](\d{1,2})$", value)
+    if m:
+        y, a, b = int(m.group(1)), int(m.group(2)), int(m.group(3))
+        d = _valid_date(y, a, b)
+        if d is None:
+            raise ValueError(f"Invalid date: {value!r}")
+        return d
+
+    # Try two-part MM-DD / DD-MM
+    m = re.match(r"^(\d{1,2})[-/](\d{1,2})$", value)
+    if m:
+        a, b = int(m.group(1)), int(m.group(2))
+        as_md = _valid_date(today.year, a, b)  # interpret as MM-DD
+        as_dm = _valid_date(today.year, b, a)  # interpret as DD-MM
+
+        if as_md is not None and as_dm is not None and as_md != as_dm:
+            raise AmbiguousDateError(
+                f"{value!r} is ambiguous: could be {as_md} (MM-DD) or {as_dm} (DD-MM)"
+            )
+        result = as_md or as_dm
+        if result is None:
+            raise ValueError(f"Invalid date: {value!r}")
+        return result
+
+    raise ValueError(
+        f"Unrecognised date format: {value!r}. "
+        "Expected YYYY-MM-DD, MM-DD, or DD-MM (- or / as separator)."
+    )
+
 
 def parse_duration(duration_str: str) -> float:
     """Convert HH:MM duration string to decimal hours."""