Make touch logic in file_metadata.sh more portable

gix-index/tests/fixtures/file_metadata.sh

@@ -1,9 +1,13 @@
 #!/usr/bin/env bash
 set -eu -o pipefail
 
-# Attempt to create files with the latest and earliest possible dates for ext4. Nanoseconds are
-# special there, but not usually on other filesystems. In some touch implementations, the format
-# may be rejected. So if a command fails, we try again with a more extreme date that is out of
-# range, because some implementations will clip it to the edge of the range (but they may fail).
-touch -d '2446-05-10 22:38:55.111111111' future || touch -d '2446-05-11 22:38:56' future
-touch -d '1901-12-13 20:45:52.222222222' past || touch -d '1901-12-13 20:45:52' past
+# Attempt to create files with the latest and earliest possible 64-bit dates/times for ext4.
+# Although nanoseconds are stored in ext4, specifying fractions of a second does not seem to make
+# this work better, and omitting them allows the commands that attempt to set these dates to
+# succeed on more systems. While we use a portable format, if the system rejects a future date as
+# out of range with an error (and touch does not automatically retry with an allowed date) then it
+# can fail. In this case, we try again with a much more moderate date: the greatest value that can
+# in practice always parse to fit within a 32-bit signed time_t. This is subject to change to
+# support changed or new tests. It will also become less useful in the near future (after 2038).
+TZ=UTC touch -d '2446-05-10 22:38:55' future || TZ=UTC touch -d '2038-01-19 03:14:07' future
+TZ=UTC touch -d '1901-12-13 20:45:52' past