Fetch an empty resource that is privately cacheable, set to max-age=0, and has an ETag containing the current timestamp and a random session id. The browser will consider its cached copy always stale.

When you next fetch that resource, because it is stale, the browser will revalidate it by passing an If-None-Match header containing the ETag. Update the ETag to include the original timestamp and the current timestamp.

So on every page load (or whichever other event you want to measure), you will be told when that session started, the session id and when that visitor was last seen.

To set the maximum session duration, reset the ETag if the last seen timestamp passed to you in If-None-Match is too long ago.

This can even work without JavaScript by using an img element.

The only data tracked with this is the session start time, last seen time, and a random session id. Since the session id isn’t related to any of your business logic, it cannot be used to identify an individual.

To further isolate this data, locate the tracking resource on a different hostname. The browser’s SOP will prevent any cookies from being sent with the request, so your analytics backend can’t record identifying information even if it wanted to. This will also prevent you from tracking which page is being visited, though you can override that with the no-referrer-when-downgrade referrer policy.

In browsers, it's as simple as:

    if (!sessionStorage.sessionReported) {
      reportSession();
      sessionStorage.sessionReported = 1;
    }