.NET の DateDiff() を使って月数を求めた。
Debug.Print(DateDiff(DateInterval.Month, CDate("2025-01-01"), CDate("2025-01-31")).ToString())
Debug.Print(DateDiff(DateInterval.Month, CDate("2025-01-01"), CDate("2025-02-01")).ToString())
Debug.Print(DateDiff(DateInterval.Month, CDate("2025-01-02"), CDate("2025-02-01")).ToString())
Debug.Print(DateDiff(DateInterval.Month, CDate("2025-01-02"), CDate("2025-02-02")).ToString())
Debug.Print(DateDiff(DateInterval.Month, CDate("2025-01-31"), CDate("2025-02-01")).ToString())
↑の結果は↓
0 1 1 1 1