Dates and timestamps

overview

JS Dates are represented by millisecond timestamps: the number of milliseconds since Jan 1st 1970, 00:00 UTC time (Unix Epoch).

read the timestamp

d_2021.getTime() // 1_609_459_200_000 (ms)
// getTimestamp(), getInstant(), getEpochMilliseconds() would be better

rely on a helper to get the current timestamp:

Date.now() // 1_771_004_506_434 (ms)

Note that the constructor with no argument creates a date with the current timestamp:

new Date().getTime()

build a Date from a timestamp:

new Date(1_627_541_982_738)
// '2021-07-29T06:59:42.738Z'
// Thu Jul 29 2021 08:59:42 GMT+0200 (Central European Summer Time)

Note : JavaScript consoles log the date in a format that is more readable than a timestamp:

• the Node.js REPL logs the date in its ISO 8601 format, in the UTC timezone
• the Chrome console uses more verbose calendar components and reads the date in the current timezone.

timestamp difference

d_1971.getTime() - d_1970.getTime() // 31 536 000 000 (ms)

compare date, check posteriority

We use the comparison operators on dates, to check posteriority. Note: TypeScript allows comparison between objects.

d2021.getTime() > d2020.getTime() // pedanctic
d2021 > d2020

(do not use) difference between Date objects do not work in TypeScript

In JS, We can subtract two Date objects, because they implement valueOf() which return their timestamp, and that is what the subtraction operates on:

d_1971 - d_1970 // 31 536 000 000 (ms)

The Typescript compiler doesn't allow it: it only sees a subtraction between objects and forbids it. It ignores the valueOf() transformation.