To echo, some of the other answers, the further back you go, the less someone cares, so it may be perfectly acceptable to estimate to the best of your knowledge (but this could depend on the individual company and may not be acceptable to all).
True story from my own experience... I applied to a company I had interned with 13 years prior (a Fortune 500 company with very strict hiring processes), and I actually had the wrong dates for the internship on my resume. I had the months correct, but I was off by an entire year. I repeated that error in my background check as well. And no one even questioned it or asked, and I was hired without issue.
In fact, I didn't even realize the mistake until I applied for an internal position 2 years later when the internal application pre-populates your internal positions.
So you should try to be as accurate as you can, but when you get past the last 10 years, it becomes less important to be 100% correct. They hiring manager is more concerned with your experience.