Date rejects valid large years that GNU date accepts

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