diff --git a/changelog.d/1531-firms-age-rounding.md b/changelog.d/1531-firms-age-rounding.md
new file mode 100644
index 000000000..cd9bcfc5d
--- /dev/null
+++ b/changelog.d/1531-firms-age-rounding.md
@@ -0,0 +1,5 @@
+- FIRMS fire ages round to whole hours before choosing a unit, so an age of
+  47.5–48 hours reads "2d", as exactly 48 hours does, not "48h". Detections
+  are filtered to the trailing 24 hours, so this shows mostly in the
+  `STALE · cached {age}` stats line after about two days without a successful
+  refresh (@quanticsoul4772, #1531).
diff --git a/docs/CURRENT-STATE.md b/docs/CURRENT-STATE.md
index 943458e5e..7e976acbb 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -3053,6 +3053,7 @@ to a set time, naming its end as `at` (epoch ms): the row then reads
 `window_end`. The dev proxy never sets `at`.
 
 The FIRMS CSV parser rejects impossible acquisition calendar dates before the proxy filters records to the trailing 24 hours, while retaining valid UTC acquisition times.
+FIRMS ages round to whole hours before choosing a unit, so a 47.6-hour age reads "2d", not "48h"; with detections filtered to 24 hours, this shows mostly in the `STALE · cached {age}` stats line.
 The `get_active_fires` tool reports acquisition times as UTC ISO timestamps,
 including zero-padded hours and minutes; missing or invalid dates and times
 remain `null`, including impossible calendar dates from a custom source.
diff --git a/src/layers/firms/model.js b/src/layers/firms/model.js
index 812f4f462..00873c2b2 100644
--- a/src/layers/firms/model.js
+++ b/src/layers/firms/model.js
@@ -535,14 +535,26 @@ export function createModel({
     return `${isolate(latPart)} ${isolate(lonPart)}`;
   }
 
+  /**
+   * Millisecond delta → `{ unit: '<1h' | 'h' | 'd', count }`, or null for
+   * invalid input. Both age formatters read it, so they pick the same unit.
+   */
+
+  function ageParts(deltaMs) {
+    if (!Number.isFinite(deltaMs) || deltaMs < 0) return null;
+    const hours = deltaMs / 3600000;
+    if (hours < 1) return { unit: '<1h', count: 0 };
+    const wholeHours = Math.round(hours);
+    if (wholeHours < 48) return { unit: 'h', count: wholeHours };
+    return { unit: 'd', count: Math.round(hours / 24) };
+  }
+
   /** Millisecond delta → "<1h" / "Xh" / "Xd", or '' for invalid input. */
 
   function formatAge(deltaMs) {
-    if (!Number.isFinite(deltaMs) || deltaMs < 0) return '';
-    const hours = deltaMs / 3600000;
-    if (hours < 1) return '<1h';
-    if (hours < 48) return `${Math.round(hours)}h`;
-    return `${Math.round(hours / 24)}d`;
+    const age = ageParts(deltaMs);
+    if (!age) return '';
+    return age.unit === '<1h' ? '<1h' : `${age.count}${age.unit}`;
   }
 
   /** Millisecond delta → "<1m ago" / "Xm ago" / "Xh ago" (fresh-feed readout). */
@@ -573,12 +585,12 @@ export function createModel({
   /** formatAge for a card: "<1h", "24h", "3d" in the UI language ("24 ч"). */
 
   function formatAgeLabel(deltaMs) {
-    if (!Number.isFinite(deltaMs) || deltaMs < 0) return '';
-    const hours = deltaMs / 3600000;
-    if (hours < 1) return t('<1h', { context: 'age' });
-    if (hours < 48)
-      return t('{count}h', { context: 'age', count: Math.round(hours) });
-    return t('{count}d', { context: 'age', count: Math.round(hours / 24) });
+    const age = ageParts(deltaMs);
+    if (!age) return '';
+    if (age.unit === '<1h') return t('<1h', { context: 'age' });
+    return age.unit === 'h'
+      ? t('{count}h', { context: 'age', count: age.count })
+      : t('{count}d', { context: 'age', count: age.count });
   }
 
   /** Normalized 0..1 confidence → low/nominal/high display bucket. */
diff --git a/src/layers/firms/model.test.mjs b/src/layers/firms/model.test.mjs
new file mode 100644
index 000000000..2c42227f8
--- /dev/null
+++ b/src/layers/firms/model.test.mjs
@@ -0,0 +1,45 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import { getLocale, setLocale } from '../../i18n/index.js';
+import { createModel } from './model.js';
+
+const HOUR_MS = 3600000;
+
+test('a fire age that rounds to 48 hours reads in days, as 48 hours does', () => {
+  const previousLocale = getLocale();
+  const { formatAge, formatAgeLabel } = createModel({
+    services: { anchors: {} },
+  });
+  try {
+    setLocale('en');
+    assert.equal(formatAge(47.4 * HOUR_MS), '47h');
+    assert.equal(formatAge(47.5 * HOUR_MS), '2d');
+    assert.equal(formatAge(47.6 * HOUR_MS), '2d');
+    assert.equal(formatAge(48 * HOUR_MS), '2d');
+    assert.equal(formatAgeLabel(47.4 * HOUR_MS), '47h');
+    assert.equal(formatAgeLabel(47.5 * HOUR_MS), '2d');
+    assert.equal(formatAgeLabel(47.6 * HOUR_MS), formatAgeLabel(48 * HOUR_MS));
+    assert.equal(formatAgeLabel(48 * HOUR_MS), '2d');
+  } finally {
+    setLocale(previousLocale);
+  }
+});
+
+test('both fire age formatters agree on invalid, sub-hour and multi-day ages', () => {
+  const previousLocale = getLocale();
+  const { formatAge, formatAgeLabel } = createModel({
+    services: { anchors: {} },
+  });
+  try {
+    setLocale('en');
+    for (const format of [formatAge, formatAgeLabel]) {
+      assert.equal(format(-1), '');
+      assert.equal(format(Number.NaN), '');
+      assert.equal(format(0.5 * HOUR_MS), '<1h');
+      assert.equal(format(1 * HOUR_MS), '1h');
+      assert.equal(format(72 * HOUR_MS), '3d');
+    }
+  } finally {
+    setLocale(previousLocale);
+  }
+});