Date range checking

[tiffany352]

Dates have no range checking which makes parsing them slightly unreliable and causes weird error messages like this:

> #25:00#
Most likely pattern `hour12:min[:sec] meridiem[ offset]` failed: Expected ` `, got eof

Additionally, large numbers cause a panic:

> #9999999999#
Panic: The application panicked (crashed).
Message:  called `Result::unwrap()` on an `Err` value: ParseIntError { kind: PosOverflow }
Location: core/src/parsing/datetime.rs:117

Backtrace omitted.
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This PR adds range checks. I also cleaned up the code a little.