Daniel Matthew

Checking Date Equality in JavaScript

Checking whether one date is equal to another is made difficult because of the way object equality works in JavaScript. Object equality isn't tested by the internal value of the object, but by identity. If the date object isn't a straight copy, it isn't considered equal. The following example will never return true:

function isChristmas(dateToTest){
  var christmas = new Date("12/25/2014");
  return (dateToTest === christmas);
}

console.log(isChristmas(new Date("12/25/2014"))); // False

To make the isChristmas function work, we need to check equality in a different way. We use the getTime method that is available on Date object, and compare the values it returns. getTime returns an integer representing the number of milliseconds since midnight of the epoch: January 1st, 1970.

function isChristmas(dateToTest){
  var christmas = new Date("12/25/2014");
  return (dateToTest.getTime() === christmas.getTime());
}

console.log(isChristmas(new Date("12/25/2014"))); // True

But! If we happen to compare against a Date object that occurs on the same day, but a different hour, we'll run into trouble - because the time elapsed since the epoch will be different.

A work around here might be to check our date against the year, month and day, like so:

function isChristmas(dateToTest){
  return (dateToTest.getFullYear() === 2014) &&
  (dateToTest.getMonth() === 11) &&
  (dateToTest.getDate() === 25);
}

console.log(isChristmas(new Date("12/25/2014 12:00"))); // True

But then this is glossing over the complexities that daylight savings and timezones introduce.

TL;DR: Be aware that there are 'gotchas' when comparing dates. Use Moment.js to avoid them and make your life easier.

Daniel is a front-end developer at Talis, with an interest in accessibility, performance, and web components.