Flag Day N. A Software Change That Is Neither Forward- Nor Backward-compatible, And Which Is Costly To Make And Costly To Reverse.

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flag day n.

A software change that is neither forward-
nor backward-compatible, and which is costly to make and costly to
reverse. "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 timesharing
system to convert from the short-lived 1965 version of the ASCII
code to the 1967 version (in draft at the time); this was scheduled
for Flag Day (a U.S. holiday), June 14, 1966. See also
backward combatability.