JS Date API introduction

instants

JS Date objects represent and store instants (aka dates) as millisecond timestamps. The timestamp is the number of milliseconds since Jan 1st, 1970 at 00:00:00 UTC, aka the Unix Epoch.

read the millisecond timestamps

We read the timestamp from a date object:

d_2021.getTime() // 1,609,459,200,000 (ms)

We can read the current timestamp:

Date.now() // 1,777,109,775,765 (ms)

build a Date from a timestamp

We build a date from a given timestamp (or from the current one)

new Date(1_627_541_982_738) // 2021-07-29T06:59:42.738Z
new Date() // from current timestamp

compare dates

The difference between two timestamps, considered as an absolute value, represent the amount of time between both dates.

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

Note: if we try to subtract dates directly, it fails in TypeScript.

We can use the comparison operators on dates to check for posteriority:

d_2021 > d_2020 // true

(misc) console built-in date formatting

JavaScript consoles don't log dates as milliseconds as it's unreadable. They log dates in a calendar format:

• The Node.js REPL use the standard ISO 8601 format (in the UTC time zone).
• The browsers' console uses a lengthier format, using the device time zone.

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