are represented by seconds, what date corresponds to 0? It isn’t, as it turns out, the
birth of Christ, but an arbitrary date: January 1, 1970, 00:00:00 UTC.
As you’re probably aware, the world is divided into time zones (TZs) so that, no mat‐
ter where you are in the morning, 7 A.M. is morning and 7 P.M. is evening. Time
zones can get complicated fast, especially as you start to consider daylight saving
time. I won’t attempt to explain all the nuances of the Gregorian calendar or time
zones in this book—Wikipedia does an excellent job of that. However, it’s important
to cover some of the basics to help us understand the JavaScript
Date
object (and
what Moment.js brings to the table).
All time zones are defined as offsets from Coordinated Universal Time (abbreviated
UTC—refer to Wikipedia for the complicated and somewhat hilarious reasons). UTC
is sometimes (and not entirely correctly) referred to as Greenwich Mean Time
(GMT). For example, I’m currently in Oregon, which is in the Pacific time zone.
Pacific time is either eight or seven hours behind UTC. Wait, eight or seven? Which is
it? Depends on the time of year. In the summer, it’s daylight saving time, and the off‐
set is seven. The rest of the year, it’s standard time, and the offset is eight. What’s
important here is not memorizing time zones, but understanding how the offsets are
represented. If I open up a JavaScript terminal, and type
new Date()
, I see the follow‐
ing:
Sat Jul 18 2015 11:07:06 GMT-0700 (Pacific Daylight Time)
Note that in this very verbose format, the time zone is specified both as an offset from
UTC (
GMT-0700
) and by its name (
Pacific Daylight Time
).
In JavaScript, all
Date
instances are stored as a single number: the number of millisec‐
onds (not seconds) since the Unix Epoch. JavaScript normally converts that number
to a human-readable Gregorian date whenever you request it (as just shown). If you
want to see the numeric representation, simply use the
valueOf()
method:
const
d
=
new
Date
();
console
.
log
(
d
);
// formatted Gregorian date with TZ
console
.
log
(
d
.
valueOf
());
// milliseconds since Unix Epoch
Constructing Date Objects
The
Date
object can be constructed in four ways. Without any arguments (as we’ve
seen already), it simply returns a
Date
object representing the current date. We can
also provide a string that JavaScript will attempt to parse, or we can specify a specific
(local) date down to the millisecond. Here are examples:
// all of the below are interpreted with respect to local time
new
Date
();
// current date
// note that months are zero-based in JavaScript: 0=Jan, 1=Feb, etc.
220 | Chapter 15: Date and Time
