Within the text of the Bhagavat Gītā itself, Lord Shri Krishna states that the knowledge of Yoga contained in the Gītā was first instructed to mankind at the very beginning of their existence. Although the original date of composition of the Bhagavad Gita is not clear, its teachings are considered timeless and the exact time of revelation of the scripture is considered of little spiritual significance.
You will find that the Bhagavat Gita is a unique book. If you truly consider its work as the written word of the Lord, then you will / should realise that its meaning is eternal and never-ending. That is to say, it will always provide you an answer and the answer will always be correct – eternally, for how can the word of the Almighty be limited!