[gottostartsomewhere]

Summary

_dts_in_same_interval(dt1, dt2, "1mo") in yfinance/utils.py compared only dt1.month == dt2.month, so two timestamps that share the same calendar month but are in different years were treated as falling in the same monthly interval. For example, (2024-12-15, 2025-12-15) returned True.

The "3mo" branch already handles cross-year correctly (see the year_diff calculation just below). This PR brings the "1mo" branch in line by also requiring dt1.year == dt2.year.

# before
elif interval == "1mo":
    last_rows_same_interval = dt1.month == dt2.month
# after
elif interval == "1mo":
    last_rows_same_interval = dt1.month == dt2.month and dt1.year == dt2.year

The function is called from fix_Yahoo_returning_live_separate (yfinance/utils.py), which merges the last two rows of a price series when they fall in the same interval, so the old behaviour could merge rows that are actually a full year apart in 1-month-interval data.

Test plan

Added test_same_month_different_year in tests/test_utils.py covering (Dec 2024, Dec 2025) and (Jun 2020, Jun 2025). It fails on the old logic and passes on the fix.

• python -m unittest tests.test_utils.TestDateIntervalCheck.test_same_month_different_year — fails before the fix, passes after.
• python -m unittest tests.test_utils -v — all 21 tests pass (20 pre-existing + 1 new), one pre-existing expected failure unchanged.
• Diff limited to the two intended lines of logic + the new test.