date: `--date` doesn't recognize negative timestamps

[cakebaker]

GNU date allows negative timestamps whereas uutils date fails:

$ LC_ALL=C date --date="@-1"
Thu Jan  1 00:59:59 CET 1970

$ cargo run date --date="@-1"
date: invalid date '@-1'

[cakebaker]

Probably fixed in uutils/parse_datetime#43, though its changes are not included in the current version 0.5.0 of parse_datetime.

[sylvestre]

@cakebaker should we make a new release?

[cakebaker]

@sylvestre Yes, it has been a while since the latest release. And there is also #6137 asking for a release.

[cakebaker]

Fixed with #6423