if you were wondering, apparently the answer is
if test ! -f build/.mtime; then printf '%b' '\n' > build/.mtime; fi; touch -r "$FILE" build/.mtime; TZ=UTC0 diff -u build/.mtime /dev/null | sed '1!d;s/.*\t\([^ ]\) \([^ ]\).*/\1T\2Z/'
(replacing \t with a tab character)
BUT this requires version 7 of POSIX
BUT it should still work in e·g Macintosh even tho they’re not strictly conformant
anyway an L·L·M would never give you this code and if it did you wouldn’t trust it