Grafana Loki HTTP API parameter "since"

Question

I'm doing Grafana Loki HTTP API queries, specifically query_range ones which produce stream responses. The parameters listed for such queries are:

1. query
2. limit
3. start
4. end
5. since
6. direction

I've got 5 of the 6 working:

const url = new URL('http://localhost:3100/loki/api/v1/query_range');

/* query */
url.searchParams.set('query', '{job="bob_job", branch="master"}');
/* limit */
url.searchParams.set('limit', '500');
/* start */
let start = new Date();
start.setHours(start.getHours() - 4);
url.searchParams.set('start', start.toISOString());
/* end */
let end = new Date();
end.setHours(end.getHours() - 2);
url.searchParams.set('end', end.toISOString());
/* since */
// url.searchParams.set('since', '5m');
/* direction */
url.searchParams.set('direction', 'forward');

console.log(url.toString());
fetch(url.toString(), {
    method: 'POST',
})
    .then((response) => response.json())
    .then((obj) => {
        console.log(obj);
    })
    .catch(console.error);

But since doesn't work. (Even if I comment out start and end.) I add it, and there's no error message. There's just no effect.

Moreover, I'm hard-pressed to find examples of other people's queries using the since parameter. What's up with it?