About the GYMD code


The GYMD code consists of four characters representing, as the name suggests, the Generation, Year, Month and Day.

Each character is chosen from a range of 36 printable characters: the numbers 0 to 9, with the (upper case) alphabet, A representing 10, B 11 and so on to Z for 35. This extends the convention used for hexadecimal numbers. The letter Z is reserved to enable a generation, year, month or day to be recorded as unknown. The remaining 35 available characters allow any day from a span of over a thousand years to be recorded (from AD 1080 to 2129). Crucially, sorting date codes puts the dates into correct chronological order.

The Characters

G The first coded character, for the generation, makes use of an existing standard, known as the Leeson generation code, put forward by Frank Leeson of The Surname Archive some time ago. This splits the years up into realistic generations of 30 years, with the letter S (which originally stood for 'Self') representing the generation 1920-1949. Generations before the letter A in the Leeson code made use of negative letters which would have required two characters, so the GYMD code instead uses numbers as above.

Y The year character codes the years 0 to 29 within a generation, so that for instance a code with 'S3' as the first two characters refers to the year 1923 (= 1920 + 3).

M The month character codes the twelve months January to December in order. However the year has not always ended in December. In England previous to the change of calendar in 1752, January, February and March (to the 24th) came at the end of the year. This would result in these dates being sorted wrongly. By using the next three codes (D, E and F) for these months when pre-1752, correct sorting order is maintained. For total generality, a whole second set of months is provided to allow for any possible year-ends in other countries and historical periods.

D The day character simply codes the days of the month in order. The whole date can be flagged as approximate though, by using the reserved letter Y in the day character position, where an exact day number wouldn't be known anyway.


GYMD is now version 2, and is freely available, though obviously applications using it should stick strictly to the code table - that is the point of having a standard.


The Leeson generation code was referred to in Computers in Genealogy, V1 No 3 (Mar 1983). In the same volume, Mr P.Cardew suggested the use of duplicate January, February and March codes for correct sorting of years before the calendar change in England.

The first reference to the GYMD code (in its original version) was in Computers in Genealogy, V1 No 8 (Jun 1984).

A useful source of information is Dates and Calendars for Genealogists by Cliff Webb, available from the Society of Genealogists.

Go to the Code Table, or back to the GYMD Home Page.