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Adding time with vanilla JavaScript

Today, we’re going to look at how to add and subtract time with vanilla JS. Let’s dig in!

The trouble with time

The current way to manage dates and times in JavaScript is with the Date() object. It’s pretty ok, but it’s not great.

It has methods for creating a Date() object from a string, formatting dates and times, and getting back specific values. But it lacks native methods to add or subtract time from a date. Timezone support is terrible, too.

There’s a new API in the works: Temporal(). It will allow you to do things like this:

// This is NOT currently supported in browsers
// Do NOT try to use this in production today
Temporal.Now.instant().add({ hours: 5, seconds: 20 });

It’s currently a stage 3 proposal, which means it’s pretty far along the standards process. But there’s no official “this is coming to browsers on {DATE}” for it, so for now, we’re stuck with Date().

So, let’s look how to add time “the old fashioned way.”

Creating a Date() object

You can create a Date() object using the new Date() constructor.

If you pass in no arguments at all, it will create a Date() object for the current instant that it’s run. You can alternatively pass in a date string as an argument.

The resulting Date() object is relative to your current timezone.

// A date object for right now
let now = new Date();

// A date object for Halloween
let halloween = new Date('October 31, 2021');

You can also create a date by passing in a series of arguments: year, monthIndex, day, hours, minutes, seconds, and milliseconds. Only year and monthIndex are required.

The monthIndex argument is confusing AF, because it starts with January at 0 instead of 1.

// Notice the month index is 9 even though October is the 10th month
let halloween = new Date(2021, 9, 31);

// Christmas morning at 10:30 am, local time
let christmas = new Date(2021, 11, 25, 10, 30);

It’s clunky, but functional.

Modifying dates

The Date() object provides a handful of instance methods that you can use to get and modify the date and time. They follow a get*() and set*() naming convention, where * is the property to get or set.

For example, to get the hours and month properties for christmas, you would do this.

// returns 10

// returns 11, because it starts with January at 0
// This is seriously really annoying!

To set the hours and month properties, you would do this, passing in the hours or month to set the Date() to, respectively. It uses a 24-hour clock for hours.

// Set the hour to 2pm, 14 hours

// Set the month to July
// (that index thing again)

Adding and removing time

Now, to add and remove time, we can use the get*() and set*() methods in conjunction.

For example, to add 4 hours to a time, you would get the current number of hours with the getHours() method, add 4 to it, and pass the resulting number into the setHours() method.

// Set the hours to 6pm (2pm + 4 hours)
christmas.setHours(christmas.getHours() + 4);

If the resulting adjustments would roll you into the next day, month, year, and so on, the Date() object automatically accounts for that and adjusts itself correctly.

// Set the date to December 26 at 2am (December 25 at 6pm + 8 hours)
// (Pretend I never modified the month)
christmas.setHours(christmas.getHours() + 8);

Formatting dates

Once you’ve modified your date and time, you can format it into a string.

There are two ways to do that: the Date.toLocaleString() method and the Intl.dateTimeFormat() method.

They both support the same formatting options. The Date.toLocaleString() method is older, and, in my opinion, a bit easier to work with. The Intl.dateTimeFormat() method is better for performance if you’re formatting lots of dates with the same settings.