Ahhh, I think the author mixed up two values here. That value seems to actually be the average over the past 3 months.

    const incidents = e.detail.incidents;
    
    // ...snip...
    
    const now = new Date();
    const threeMonthsAgo = new Date(now);
    threeMonthsAgo.setMonth(threeMonthsAgo.getMonth() - 3);
    
    // ...snip...
    
    var currentFreq = recentIncidents.length / 3; // <- We out here, smoking these guns with our homeboy Claude
    
    // ...snip...
    
    var earliest = null;
    for (var j = 0; j < incidents.length; j++) {
      var dd = new Date(incidents[j].started_at); // <- eventually incidents[j].started_at is "2016-03-01T07:07:37.000Z"
      if (!earliest || dd < earliest) earliest = dd;
    }
    
    // ...snip...
    
    document.getElementById('n-since').textContent = earliest
      ? earliest.toLocaleDateString('en-US', { month: 'long', year: 'numeric' })
      : '?';
    document.getElementById('n-rate').textContent = Math.round(currentFreq * 10) / 10;

#n-rate has nothing to do with the #n-since month, it's just the last trailing 3 months. And even then, it's sort of underbaked? It's moving the date back by 3 calendar months not taking into account differing numbers of days, so it'll under-report short months.

I wouldn't trust the stats here.

Edit: whoops, author updated the template while I was writing this! It now says "Over the last 3 months", though that's still calendar months.