Date rejects valid large years that GNU date accepts

[sylvestre]

GNU:

  $ LANG=C TZ=UTC0 /usr/bin/date -d "18978-01-01"
  Thu Jan  1 00:00:00 UTC 18978

Rust:

  $ LANG=C TZ=UTC0 ./target/debug/date -d "18978-01-01"
  date: invalid date '18978-01-01'

• GNU date: Accepts year 18978 and outputs the date correctly
• Rust date: Rejects year 18978 as invalid date, appears to have a limit around year 9999
• GNU date supports much larger year ranges than our implementation

Found with a fuzzer

[cakebaker]

See also uutils/parse_datetime#160

[n22arora]

can I work on this?

[cakebaker]

@n22arora sure, go ahead! However, I'm not sure whether this is a good first issue as it looks like it's a limitation of the library we use :|