Safari history, using a specific date (April 9th, which I believe is my login date) shows Facebook searches which are months old. How is this possible?
One thing I could possibly think of is the wrong date on your computer. Make sure that it's adjusted properly because this could also influence the search history. Also, your account or computer might be compromised or someone else from your family used the account and searched for various things (as shown in the search history). There are various actions you can do this time:
Secure your Facebook account with a new password and enable 2 Factor Authentication.
Check the clock settings to adjust them as necessary.
Ask around your family to see who used the account.
If the computer has been infected, run an antivirus solution. You can try BitDefender.