She was an idiot savant who couldn't navigate a grocery store but could mentally calculate the number of days between any two dates.
idiot savant = savant with mental disability
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Mr. Dalton says he's somewhat of an idiot savant, which mean he's a stupid genius!†
(source)
Darren Shan, A Living Nightmare, 2001
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We fall into clans: Jocks, Country Clubbers, Idiot Savants, Cheerleaders, Human Waste, Eurotrash, Future Fascists of America, Big Hair Chix, the Marthas, Suffering Artists, Thespians, Goths, Shredders.†
(source)
Laurie Halse Anderson, Speak, 1999