Using relative specifier with "at time" should fail

[cakebaker]

While going through the examples in #84 I noticed that uutils date accepts relative specifiers like "yesterday" with "at time" whereas GNU date fails. Some examples:

$ date -d "today at 10:00"
date: invalid date 'today at 10:00'
$ date -d "tomorrow at 8pm"
date: invalid date 'tomorrow at 8pm'
$ date -d "yesterday at 8"
date: invalid date 'yesterday at 8'

$ cargo run -q date -d "today at 10:00"
Mon Aug  4 16:00:00 CEST 2025
$ cargo run -q date -d "tomorrow at 8pm"
Wed Aug  6 02:00:00 CEST 2025
$ cargo run -q date -d "yesterday at 8"
Mon Dec 31 04:34:08 LMT 0007

[yuankunzhang]

The at was parsed as a time zone. Ref:

parse_datetime/src/items/time.rs

Line 545 in 46a5ef8

"at" => Ok("-4:00"),

Apparently GNU date doesn't support this many named time zones as we do here. We need to go through the time zone list to remove the ones that are not supported by GNU date.