Function serialize

pub fn serialize<S>(
    opt: &Option<DateTime<Utc>>,
    serializer: S,
) -> Result<S::Ok, S::Error>
where
    S: Serializer,

Serialize a UTC datetime into an integer number of milliseconds since the epoch or none

Intended for use with serdes serialize_with attribute.

Example:

use chrono::serde::ts_milliseconds_option::serialize as to_milli_tsopt;
#[derive(Serialize)]
struct S {
    #[serde(serialize_with = "to_milli_tsopt")]
    time: Option<DateTime<Utc>>,
}

let my_s = S {
    time: Some(
        NaiveDate::from_ymd_opt(2018, 5, 17)
            .unwrap()
            .and_hms_milli_opt(02, 04, 59, 918)
            .unwrap()
            .and_utc(),
    ),
};
let as_string = serde_json::to_string(&my_s)?;
assert_eq!(as_string, r#"{"time":1526522699918}"#);