Add now.js: recompute "Last updated" relative phrase client-side

build/Now.hs renders the .now-stamp-relative phrase ("3 days ago") at
build time against the build machine's clock; a page served days later
from cache or a CDN would then read stale. now.js recomputes the
phrase in the browser from the <time datetime> attribute (an
unambiguous YYYY-MM-DD) against the visitor's clock, with bucket
thresholds that mirror Now.hs:relativeTime exactly so the no-JS
fallback and the recomputed value agree.

* static/js/now.js — the recomputation script.
* templates/default.html includes it via $if(now)$ so it only loads
  on the Current page.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

static/js/now.js

@@ -0,0 +1,75 @@
+/* now.js — Keep the Current page's "Last updated" relative phrase
+   honest.
+
+   build/Now.hs renders `.now-stamp-relative` ("3 days ago") at build
+   time, relative to the build machine's clock. A page served days
+   later from cache/CDN would then lie. We recompute the phrase in the
+   browser from the `<time datetime>` attribute (an unambiguous
+   YYYY-MM-DD), against the visitor's own clock.
+
+   The bucket thresholds below mirror `relativeTime` in build/Now.hs
+   exactly — keep the two in sync. The server-rendered text remains the
+   no-JS fallback and is only replaced once we've recomputed. */
+(function () {
+    'use strict';
+
+    function relative(days) {
+        if (days < 0)   return '';            // future / clock skew
+        if (days === 0) return 'today';
+        if (days === 1) return 'yesterday';
+        if (days < 7)   return days + ' days ago';
+
+        var n, unit;
+        if (days < 28)       { n = Math.floor(days / 7);   unit = 'week';  }
+        else if (days < 365) { n = Math.floor(days / 30);  unit = 'month'; }
+        else                 { n = Math.floor(days / 365); unit = 'year';  }
+        return n === 1 ? ('1 ' + unit + ' ago')
+                       : (n + ' ' + unit + 's ago');
+    }
+
+    function update() {
+        var stamp = document.querySelector('.now-stamp');
+        if (!stamp) return;
+
+        var timeEl = stamp.querySelector('.now-stamp-date');
+        if (!timeEl) return;
+
+        var iso = timeEl.getAttribute('datetime');
+        var m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(iso || '');
+        if (!m) return;   // unparseable — leave the SSR fallback as-is
+
+        // Calendar-day difference, computed via UTC epoch days so DST
+        // transitions can't add or drop a day. "today" uses the
+        // visitor's *local* date components, matching what they'd
+        // read off a wall calendar.
+        var then = Date.UTC(+m[1], +m[2] - 1, +m[3]);
+        var local = new Date();
+        var today = Date.UTC(
+            local.getFullYear(),
+            local.getMonth(),
+            local.getDate()
+        );
+        var days = Math.round((today - then) / 86400000);
+        var text = relative(days);
+
+        var rel = stamp.querySelector('.now-stamp-relative');
+        if (!text) {
+            // No meaningful relative phrase (e.g. dated in the future):
+            // drop any stale server-rendered one rather than keep a lie.
+            if (rel) rel.remove();
+            return;
+        }
+        if (!rel) {
+            rel = document.createElement('span');
+            rel.className = 'now-stamp-relative';
+            stamp.appendChild(rel);
+        }
+        rel.textContent = text;
+    }
+
+    if (document.readyState === 'loading') {
+        document.addEventListener('DOMContentLoaded', update);
+    } else {
+        update();
+    }
+})();

templates/default.html

@@ -29,6 +29,7 @@ $partial("templates/partials/footer.html")$
 <script src="/js/lightbox.js" defer></script>
 $if(home)$<script src="/js/random.js" defer></script>$endif$
 $if(reading)$<script src="/js/reading.js" defer></script>$endif$
+$if(now)$<script src="/js/now.js" defer></script>$endif$
 $if(photography)$<script src="/js/photography-modes.js" defer></script>$endif$
 $if(photography-map)$<script src="/leaflet/leaflet.js" defer></script>$endif$
 $if(photography-map)$<script src="/leaflet/leaflet.markercluster.js" defer></script>$endif$