Since the 5-byte (rather than 8-byte) extension to the time zone binary data file format went over with an overwhelming thud, my current thinking is to stick revert to the 8-byte approach. The name of the game would be to produce binary files with: 1. data with 32-bit time values through 2037 (for use by old systems) 2. data with 64-bit time values for "historic" years 3. a newline enclosed, POSIX-conforming TZ-environment-variable-style string telling what to do in years beyond those covered by 2 above. There are a few places (Cairo, Godthab, Chile) that can't be represented by POSIX-conforming strings; for these we'd punt to the "write 400 years worth of data and work modulo 400" approach (leaving off the trailing TZ string). --ado