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-since is going to be either march or feburary. It'll change depending on your timezone because JS's Date object always shifts the date around to match the same instant but in the system's timezone.

#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.