fix(common/time_utils): correct None/empty timestamp fallback and ISO 8601 parsing (#16483)

Recovery PR for #16173 after the fork branch was accidentally reset
during rewrite-cleanup.

Cherry-picked onto current `main`:
- fix(common/time_utils): correct fallback timestamp and ISO-8601
normalization
- fix(common/time_utils): preserve zero timestamps and mark regression
tests
- test(common/time_utils): make fallback assertions deterministic

Supersedes closed #16173 — same branch
`Harsh23Kashyap/fix/time-utils-edgecases`, rebuilt per @yuzhichang
recovery steps in
https://github.com/infiniflow/ragflow/pull/16173#issuecomment-4829663835

---------

Co-authored-by: Harsh Kashyap <harshkashyap@Harshs-MacBook-Pro.local>
Co-authored-by: Cursor <cursoragent@cursor.com>

test/unit_test/common/test_time_utils.py

@@ -17,7 +17,7 @@
 import time
 import datetime
 import pytest
-from common.time_utils import current_timestamp, timestamp_to_date, date_string_to_timestamp, datetime_format, delta_seconds
+from common.time_utils import current_timestamp, timestamp_to_date, date_string_to_timestamp, datetime_format, delta_seconds, format_iso_8601_to_ymd_hms
 
 
 class TestCurrentTimestamp:
@@ -650,4 +650,65 @@ class TestDeltaSeconds:
             # If we're testing on the first day of month
             date_string = "2024-01-31 12:00:00"  # Use a known past date
             result = delta_seconds(date_string)
-            assert result > 0
+            assert result > 0
+
+
+@pytest.mark.p2
+class TestTimestampToDateCurrentTimeFallback:
+    """Regression tests for the None/empty fallback of timestamp_to_date.
+
+    The docstring promises "If None or empty, uses current time", but the
+    fallback assigned ``time.time()`` (seconds) and then divided by 1000 again,
+    producing a date around 1970-01-21 instead of now. The existing
+    ``test_return_type_always_string`` only checks the return type, so it never
+    caught this. These tests pin the behaviour by value.
+    """
+
+    def test_none_uses_current_time(self, monkeypatch):
+        """None input must resolve to current_timestamp() fallback."""
+        fixed_ms = 1704067200123
+        monkeypatch.setattr("common.time_utils.current_timestamp", lambda: fixed_ms)
+        assert timestamp_to_date(None) == time.strftime(
+            "%Y-%m-%d %H:%M:%S", time.localtime(fixed_ms / 1000)
+        )
+
+    def test_empty_string_uses_current_time(self, monkeypatch):
+        """Empty-string input must resolve to current_timestamp() fallback."""
+        fixed_ms = 1704067200123
+        monkeypatch.setattr("common.time_utils.current_timestamp", lambda: fixed_ms)
+        assert timestamp_to_date("") == time.strftime(
+            "%Y-%m-%d %H:%M:%S", time.localtime(fixed_ms / 1000)
+        )
+
+    def test_zero_timestamp_is_not_treated_as_empty(self):
+        """Zero timestamp should map to Unix epoch, not fallback to current time."""
+        assert timestamp_to_date(0) == time.strftime("%Y-%m-%d %H:%M:%S", time.localtime(0))
+
+
+@pytest.mark.p2
+class TestFormatIso8601ToYmdHms:
+    """Test cases for format_iso_8601_to_ymd_hms function."""
+
+    def test_standard_utc_z(self):
+        """Standard UTC timestamp with trailing Z."""
+        assert format_iso_8601_to_ymd_hms("2024-01-01T12:00:00Z") == "2024-01-01 12:00:00"
+
+    def test_explicit_utc_offset(self):
+        """Timestamp with an explicit +00:00 offset."""
+        assert format_iso_8601_to_ymd_hms("2024-01-01T12:00:00+00:00") == "2024-01-01 12:00:00"
+
+    def test_ordinal_date_extended(self):
+        """ISO 8601 ordinal date (day-of-year), extended form.
+
+        dateutil.isoparse accepts it but datetime.fromisoformat rejects it,
+        which previously made the function silently return the input unchanged.
+        """
+        assert format_iso_8601_to_ymd_hms("2024-001T12:00:00Z") == "2024-01-01 12:00:00"
+
+    def test_ordinal_date_basic(self):
+        """ISO 8601 ordinal date (day-of-year), basic form."""
+        assert format_iso_8601_to_ymd_hms("2024001T120000Z") == "2024-01-01 12:00:00"
+
+    def test_invalid_string_returns_original(self):
+        """Unparseable input is returned unchanged."""
+        assert format_iso_8601_to_ymd_hms("not-a-date") == "not-a-date"