So, “Acquired Savant Syndrome”.
You have heard of that right? Example: person gets clunked on the head. Has a headache for a couple of days and suddenly determines that he now has a new talent. She can play the piano like Wagner or is now a math genius or suddenly understands and speak a new language that they never learned or spoke before.
Calendar calculating is a fun one. Give them the date (out of the past) and they tell you what day of the week it was. This could be fun; try giving them a date of the future and see what happens.
Child prodigies with extreme musical talent or other talent. There is no scientific (or otherwise) explanation for this phenomenon. It is presented to us, and we are to just go with it in the anticipation that one day someone will provide a logical explanation.
The answer here is more sinister than you would imagine. You see we reside in a world where there are unseen realms and dimensions in which unseen persons reside. The unseen persons from those other realms and dimensions may enter our world at will and yes may even enter earthly bodies and even take control either partially or fully depending upon the vulnerability present at the time.
Disembodied spirits are present on the earth and carry with them all sorts of agendas.