What is an idiot genius?

In the 1980s, foreign literature recorded the case of such an idiot genius? The academic performance in school is very poor, and the IQ is only 50 (90 for normal people to 1 10). However, he has an unusual ability to tell the date of any day from 1880 to 1950 almost without thinking. One of his favorite entertainments is to ask people's birthdays and then tell them their birthdays last year and next year.

However, he doesn't even have the basic ability to tell a person's age?

He can correctly add 10 to 12, but he doesn't know that 20 is greater than 8, or even what these numbers mean.

idiot genius