JavaScript code for extracting ISO 8601 dates/time fields from a JavaScript Date object.

iso8601_extract_datetimes.js

// Passing a string to new Date() is cross-implementation-safe only if the 
// format of the string conforms to ECMAScript's specified format.
// From the specification (https://tc39.es/ecma262/#sec-date.parse):
// " If the String does not conform to that format the function may fall 
// back to any implementation-specific heuristics or implementation-specific date formats. "
// As such, we are good to go here with passing it a correct ISO 8601 date/time string.

let d = new Date("2020-04-02T13:00:00.000Z")

// The beauty of a fixed-width ISO 8601 output string.
// It is a breeze to extract any given date/time field.
// isoDateTime will be formatted as: YYYY-MM-DDTHH:mm:ss.sssZ
let isoDateTime = d.toISOString()

let isoYear = isoDateTime.substr(0,4) // "2020"
let isoYearMonth = isoDateTime.substr(0,7) // "2020-04"
let isoDate =  isoDateTime.substr(0,10) // "2020-04-02"
let isoHourMinute = isoDateTime.substr(11,5) // "13:00"
let isoTime = isoDateTime.substr(11)  // "13:00:00.000Z"