flag day

jargon A software change that is neither forward- nor

backward-compatible, and which is costly to make and costly to

reverse. E.g. "Can we install that without causing a flag day

for all users?"

This term has nothing to do with the use of the word flag to

mean a variable that has two values. It came into use when a

massive change was made to the Multics time-sharing system

to convert from the old ASCII code to the new one; this was

scheduled for Flag Day (a US holiday), June 14, 1966.

See also backward combatability, lock-in.

[Jargon File]

(1998-01-15)