The MUMPS Epoch

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The MUMPS Epoch


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In MUMPS, the current date and time is contained in a special system variable, $H (short for "HOROLOG"). The format is a pair of integers separated by a comma, e.g. "54321,12345" The first number is the number of days since December 31, 1840, i.e. day number 1 is January 1, 1841; the second is the number of seconds since midnight.

The reason for this not very obvious choice of epoch is a bit of MUMPS trivia. James M. Poitras has written that he chose this epoch for the date and time routines in a package developed by his group at MGH in 1969: "I remembered reading of the oldest (one of the oldest?) U.S. citizen, a Civil War veteran, who was 121 years old at the time. Since I wanted to be able to represent dates in a Julian-type form so that age could be easily calculated and to be able to represent any birth date in the numeric range selected, I decided that a starting date in the early 1840s would be 'safe.' Since my algorithm worked most logically when every fourth year was a leap year, the first year was taken as 1841. The zero point was then December 31, 1840.... I wasn't party to the MDC negotiations, but I did explain the logic of my choice to members of the Committee."

(More colorful versions have circulated in the folklore, suggesting, for example, that December 31, 1840 was the exact date of the first entry in the MGH records, but these seem to be urban legends.)

A piece of MUMPS trivia: $HOROLOG hit 60000 on April 10, 2005; will be 70000 on August 26, 2032; 80000 on January 12, 2060; 90000 on May 30, 2087; and 100000 on October 16, 2114.

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