diff --git a/server/test/staticPartyService.test.ts b/server/test/staticPartyService.test.ts
index 98c1ca8..c2aa116 100644
--- a/server/test/staticPartyService.test.ts
+++ b/server/test/staticPartyService.test.ts
@@ -195,11 +195,15 @@ describe('StaticPartyStore', () => {
     const otherFile = path.join(partiesDir(), `${other.id}.json`);
     const past = new Date(Date.now() - 60000);
     fs.utimesSync(otherFile, past, past);
+    // Compared against what the filesystem itself reports back, not
+    // past.getTime(): Linux returns mtimeMs as a float that can land a
+    // hair off the exact millisecond set (e.g. ...780.999 for ...781).
+    const mtimeBefore = fs.statSync(otherFile).mtimeMs;
 
     StaticPartyService.markWatched(sp, alice, 'e1');
     await StaticPartyStore.writeNow();
 
-    assert.equal(fs.statSync(otherFile).mtimeMs, past.getTime(), 'the untouched party\'s file was not rewritten');
+    assert.equal(fs.statSync(otherFile).mtimeMs, mtimeBefore, 'the untouched party\'s file was not rewritten');
   });
 
   it('deleting a party removes its file, even with a save still pending', async () => {