CHAPTER 15
Date and Time
Most real-world applications involve working with date and time data. Unfortunately,
JavaScript’s
Date
object (which also stores time data) is not one of the language’s best-
designed features. Because of the limited utility of this built-in object, I will be intro‐
ducing Moment.js, which extends the functionality of the
Date
object to cover
commonly needed functionality.
It’s an interesting bit of history that JavaScript’s
Date
object was originally imple‐
mented by Netscape programmer Ken Smith—who essentially ported Java’s
java.util.Date
implementation into JavaScript. So it’s not entirely true that Java‐
Script has nothing to do with Java: if anyone ever asks you what they have to do with
each other, you can say, “Well, aside from the
Date
object and a common syntactic
ancestor, very little.”
Because it gets tedious to keep repeating “date and time,” I will use “date” to implicitly
mean “date and time.” A date without a time is implicitly 12:00 A.M. on that day.
Dates, Time Zones, Timestamps, and the Unix Epoch
Let’s face it: our modern Gregorian calendar is a fussy, overcomplicated thing, with 1-
based numbering, odd divisions of time, and leap years. Time zones add even more
complexity. However, it’s (mostly) universal, and we have to live with it.
We’ll start with something simple: the second. Unlike the complicated division of
time in the Gregorian calendar, seconds are easy. Dates and times—as represented by
seconds—are a single number, neatly ordered on a number line. Representing dates
and time in seconds is therefore ideal for computation. However, it doesn’t work so
well for human communication: “Hey, Byron, want to have lunch at 1437595200?”
(1437595200 is Wednesday, July 22, 2015, at 1 P.M. Pacific Standard Time). If dates
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