No. 65: Sep-Oct 1989
M.J.A. Howe and J. Smith have reported on an extensive study of calendar-calculating by individuals with otherwise subnormal intelligence. It is very clear that these so-called "idiot savants" use a variety of mental techniques, all rather different from rote memory, such as employed by memorizers of pi. First, we present Howe-and-Smith's abstract; then, a particularly interesting specific case.
"A number of mentally handicapped individuals are able to solve difficult calendar date problems such as specifying the day of the week for a particular date, sometimes over spans of more than 100 years. These individuals are self-taught and do not follow procedures at all similar to the usual, published, algorithms. An investigation of one individual revealed that he retained considerable information about the structure of days in particular months, probably as visual images. His skill closely depended on the extent and form of his knowledge of calendars, and his errors were often a consequence of lack of knowledge about a particular time period. Mentally retarded individuals who perform calendar date feats are often socially withdrawn and devote considerable periods of time to calendar dates. The most capable calendar-date calculators are usually individuals who have a strong interest in calendars as such."
Although some calendar calculators may use visual imagery - perhaps something like eidetic imagery - at least one calendar calculator was blind from birth.
"One of the few serious attempts that have been made to understand the mental operations underlying calendar skills is described by William Horwitz and others. They examined the abilities of a pair of mentally retarded identical male twins, both of whom performed calendar-calculating feats. During the twins' early childhood, despite severe family difficulties caused partly by the father's alcoholism, the parents, whose efforts to teach numerical and reading abilities to their sons were largely unsuccessful, were impressed by seeing one of the boys looking at a perpetual calendar in an almanac. Subsequently the parents encouraged the boys to acquire calendar skills. The feats of one twin were especially remarkable: he was reported to have had a range of at least 6000 years - beyond the range of any conventional or (socalled) perpetual calendar - although he gave incorrect answers for dates prior to 1582, owing to his ignorance of the 10-day calendar adjustment which was made at that time, when the Gregorian Calendar replaced the Julian one. As well as solving problems requesting the days of the week for specified dates, the more able twin could also answer questions asking, for instance, for those months in a given year in which the first day fell on a Friday, or the dates of specified days, such as 'The fourth Monday in February 1993?' (the 22nd)."
(Howe, Michael J.A., and Smith, Julia; "Calendar Calculating in 'Idiot Savants,' How Do They Do It?" British Journal of Psychology, 79:371, 1988.)