Get current document's Last-Modified date in javascript Date object

Question

The browser provides a way to determine a document's last-modified date by looking at document.lastModified. This property is determined from the HTTP Last-Modified header, and is returned as a string.

My goal is to convert this property to a Javascript Date object. Currently I am using

var date = new Date(document.lastModified);

which successfully parses the string. However, I am curious as to whether this will work across browsers and across locales.

What is very interesting to me is that the document.lastModified represents the same date as the HTTP Last-Modified header given, but the strings are not identical. It seems to me that the browser parses the Last-Modified header, converts it to its internal date representation, and then sets document.lastModified to a string based on that. If this is the case, document.lastModified is likely to be formatted in a way such that it can be parsed by the Javascript Date constructor, as they are both likely using the same locale and formatting rules. But I've been unable to confirm this for sure.

Answer 1

Here is an example:

<!--
        function makeArray() {
          for (i = 0; i < makeArray.arguments.length; i++)
            this[i] = makeArray.arguments[i];
        }

        function getFullYear(d) {
          var y = d.getYear();
          if (y < 1000) {
            y += 1900
          };
          return y;
        }

        var days = new makeArray("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday");
        var months = new makeArray("January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December");

        function format_time(t) {
            var Day = t.getDay();
            var Date = t.getDate();
            var Month = t.getMonth();
            var Year = getFullYear(t);
            timeString = "";
            timeString += days[Day];
            timeString += " ";
            timeString += months[Month];
            timeString += " ";
            timeString += Date;
            timeString += ", ";
            timeString += Year;
            return timeString;
          }
// -->

        m = new Date(document.lastModified);
        d = new Date();
        $(function() {
          $('.timestamp').html(format_time(m))
        });

.timestamp-wrap { font-size : 22px; font-family : 'Open Sans'; }
.timestamp { color: green; }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="timestamp-wrap">
  Updated on <span class="timestamp"></span>
</div>

Answer 2

Try this ? `

function getLastMod() {
    var lastMod = Date.parse(document.lastModified);
    var now = new Date();
    var diff = (new Date(now.toLocaleString())).getTimezoneOffset() - now.getTimezoneOffset();
    if (!diff) diff = now.getTimezoneOffset() * 60000;
    else diff = 0;
    return lastMod - diff;
}

`