JavaScript document.lastModified: Read a Page Modification Time

document.lastModified returns a string containing the date and local time that the browser reports for the current document's last modification.

const modified = document.lastModified;
console.log(modified);

Read document.lastModified Top ↑

const value = document.lastModified;
console.log(typeof value); // "string"

The original tutorial used document.write() to display this value. For normal page output, assign the text to an existing element instead.

Convert the Value to a Date Object Top ↑

When the string is parseable, create a Date object:

const modifiedDate = new Date(document.lastModified);

if (!Number.isNaN(modifiedDate.getTime())) {
  console.log(modifiedDate);
}

See the JavaScript Date object tutorial for date methods and formatting.

Format the Modification Date Top ↑

The legacy example manually joined day, month, and year and accidentally used the zero-based month directly. A safer display is:

const modifiedDate = new Date(document.lastModified);
const formatted = modifiedDate.toLocaleString();
console.log(formatted);

What document.lastModified Can and Cannot Tell You Top ↑

This value depends on information available to the browser. It may reflect server response metadata or file information, but it is not a guaranteed content-version system and should not be treated as authoritative publishing metadata.

For a site that needs reliable content dates, store and render those dates from the content or server-side publishing system rather than inventing them from the browser value.

Compare Parsed Times, Not Display Strings Top ↑

const modifiedMs = Date.parse(document.lastModified);

if (!Number.isNaN(modifiedMs)) {
  console.log(modifiedMs);
}

Comparing timestamps is more reliable than comparing formatted date strings.

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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