diff --git a/changelog.d/1531-firms-age-rounding.md b/changelog.d/1531-firms-age-rounding.md
index e8cc12c4b..cd9bcfc5d 100644
--- a/changelog.d/1531-firms-age-rounding.md
+++ b/changelog.d/1531-firms-age-rounding.md
@@ -1,3 +1,5 @@
-- FIRMS fire ages round to whole hours before choosing a unit, so a detection
-  47.5–48 hours old reads "2d", as one exactly 48 hours old does, not "48h"
-  (@quanticsoul4772, #1531).
+- 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 726201138..1e1230833 100644
--- a/docs/CURRENT-STATE.md
+++ b/docs/CURRENT-STATE.md
@@ -2382,7 +2382,7 @@ reporting are unchanged.
 ## FIRMS source status
 
 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".
+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.test.mjs b/src/layers/firms/model.test.mjs
index 7ff12cf3f..a0df2eb2c 100644
--- a/src/layers/firms/model.test.mjs
+++ b/src/layers/firms/model.test.mjs
@@ -13,9 +13,11 @@ test('a fire age that rounds to 48 hours reads in days, as 48 hours does', () =>
   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 {