Using `chrono`'s `serde` feature uses ISO 8601 instead of a Unix
timestamp as specified in RFC 7519 section 2, "NumericDate". This
example uses custom de/serialize functions as shown in the [serde.rs
example, "Custom Date
Format"](https://serde.rs/custom-date-format.html).
NOTE: Currently fractional values are not supported in the example,
though they are in the spec.
Kellen Frodelius-Fujimoto