JavaScript Dates
Dates can be compared and subtracted because they are numbers underneath.
Subtracting two dates gives the difference in milliseconds.
Dividing that number converts it to days, hours or minutes.
Example
javascript
const start = new Date(Date.UTC(2026, 0, 1));
const end = new Date(Date.UTC(2026, 0, 15));
const days = (end - start) / (1000 * 60 * 60 * 24);
console.log(days);The output is 14.
Subtracting dates gives milliseconds.
One day is 1000 times 60 times 60 times 24 milliseconds.
Comparison operators work directly on dates.
Syntax
javascript
const ms = laterDate - earlierDate;
const days = ms / 86400000;=== does not work; compare timestamps instead.
setUTCDate rolls over into the next month by itself.
Example
javascript
const date = new Date(Date.UTC(2026, 0, 30));
date.setUTCDate(date.getUTCDate() + 5);
console.log(date.getUTCMonth());
console.log(date.getUTCDate());January 30 plus 5 days becomes February 4.
Less than and greater than work as expected.
=== does not, because two Date objects are never identical.
Example
javascript
const a = new Date(Date.UTC(2026, 0, 1));
const b = new Date(Date.UTC(2026, 0, 15));
const c = new Date(Date.UTC(2026, 0, 1));
console.log(a < b);
console.log(a === c);
console.log(a.getTime() === c.getTime());Compare getTime() to test for the same moment.
Divide the difference and round to handle daylight saving.
Example
javascript
function daysBetween(a, b) {
return Math.round((b - a) / 86400000);
}
const start = new Date(Date.UTC(2026, 0, 1));
const end = new Date(Date.UTC(2026, 1, 1));
console.log(daysBetween(start, end));January has 31 days, so that is the answer.
The setters change the date in place, which is easy to forget.
Example
javascript
const original = new Date(Date.UTC(2026, 0, 1));
const later = new Date(original.getTime());
later.setUTCDate(later.getUTCDate() + 10);
console.log(original.getUTCDate());
console.log(later.getUTCDate());The output is 1 then 11.
Adding a month to the 31st can land in the following month.
This is where a date library starts to earn its place.
Example
javascript
const date = new Date(Date.UTC(2026, 0, 31));
date.setUTCMonth(date.getUTCMonth() + 1);
console.log(date.getUTCMonth());
console.log(date.getUTCDate());February has no 31st, so it rolled into March.
Complete Example
html
<!DOCTYPE html>
<html>
<head>
<title>JavaScript Date Arithmetic</title>
</head>
<body>
<h1>Date Arithmetic</h1>
<p id="out"></p>
<script>
const start = new Date(Date.UTC(2026, 0, 1));
const end = new Date(Date.UTC(2026, 1, 1));
const days = Math.round((end - start) / 86400000);
const later = new Date(start.getTime());
later.setUTCDate(later.getUTCDate() + 10);
document.getElementById("out").innerHTML =
"Days between: " + days +
"<br>Ten days later: " + later.toISOString().slice(0, 10) +
"<br>Start is earlier: " + (start < end);
</script>
</body>
</html>Run the above example in the Try It Editor.
Try a different gap:
Change the end month and watch the day count change.
< and > work on dates directly.=== does not; compare getTime().Date arithmetic works because a date is a number underneath.
Comparing with === is the mistake people make first.
Month arithmetic is genuinely awkward, and a library helps.