Calendar calculation: A systematic review of 100 years of research

Calendar calculation is the ability to answer rapidly to questions such as "What day of the week was May 12, 1978?" or "For which years is February 15 a Monday?" This ability is mastered by some "savant" autistic people with a surprising level of speed and accuracy. The quasi-specificity of calendar calculation in autism justifies its importance for understanding autistic information processing and learning mechanisms and is informative of certain extreme possibilities of human cognition. A registered (PROSPERO: CRD42021254855) systematic review was conducted using PRISMA guidelines, generating 76 articles (1920-2023) documenting 105 calculators (95 M). We examine: the clinical characteristics of calculators, their cognitive performances, the development and the behavioral correlates to the ability, the empirical findings on calendar calculation, as well as the overall available brain imaging results during calendar tasks. Our findings indicate that calendar calculation is associated with autism and is typically acquired implicitly and in an autodidactic manner, often during school age. Participants tend to demonstrate superior cognitive abilities in their area of interest compared to other domains. When assessed using standardized tools, their performance generally falls within the low full-scale IQ range. 49.5 % had a total calculation range under 100 years. Distance and priming effect were not consistently found showing performance variability. Brain imaging results highlighted three different neural networks that were activated during calendar tasks: memory, visual and arithmetic. This knowledge enables us to establish the common characteristics of calendar calculators and identify gaps in knowledge related to the acquisition of calendar calculation.