How Can I Calculate the Time Difference Between Two Dates in JavaScript?

Calculating the Time Difference Between Two Dates in JavaScript

In JavaScript, you can leverage the getTime() method or simply use a date in a numeric expression to convert it to milliseconds since the Unix epoch. To obtain the time difference, simply subtract the two dates.

To create a new date based on the difference, pass the number of milliseconds to the constructor. Here's an example:

var oldBegin = ...
var oldEnd = ...
var newBegin = ...

var newEnd = new Date(newBegin + oldEnd - oldBegin);

This operation should yield the desired result.

Explanation:

Date instances (oldBegin, oldEnd, and newBegin) trigger JavaScript auto casting when used with operators like and -. This triggers a call to the valueOf() method, which for Date objects returns the number of milliseconds since the epoch.

Therefore:

date.getTime() === date.valueOf() === (0 + date) === (+date)

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