+### `diff`
+
+This is a Posix utility, but ⛩️📰 书社 depends on functionality
+ introduced after `POSIX.1-2001` (the `-u` option, introduced in
+ `POSIX.1-2008`).
+Macintosh systems somewhat interestingly implement this option
+ correctly in legacy mode (`COMMAND_MODE=legacy`) but incorrectly by
+ default (despite claiming `POSIX.1-2008` conformance for this
+ utility).
+[Note this erroneous comment claiming nanosecond & timezone are
+ extensions rather than standardized.][rdar-92753335]
+Despite this, the default Macintosh implementation will still work with
+ ⛩️📰 书社, with the caveat that the timestamp will only include a
+ fractional component when a Posix⹀compliant (e·g, Macintosh legacy or
+ G·N·U) implementation is used.
+