diff --git a/.vscode/launch.json b/.vscode/launch.json
index b461c8da2..1d1b45292 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -35,6 +35,9 @@
       "runtimeArgs": [
         "--inspect=9229"
       ],
+      "env": {
+        "LANG": "en-US"
+      },
       "args": [
         "--exit",
         "-u",
@@ -43,6 +46,8 @@
         "--diff",
         "-r",
         "ts-node/register",
+        "-r",
+        "test/mocha.setup.js",
         "test/pure-tests/**/*.ts"
       ],
       "stopOnEntry": false,
diff --git a/extensions/ql-vscode/package.json b/extensions/ql-vscode/package.json
index 226a7e1ac..6f1fb1c5c 100644
--- a/extensions/ql-vscode/package.json
+++ b/extensions/ql-vscode/package.json
@@ -1188,7 +1188,7 @@
     "watch:extension": "tsc --watch",
     "watch:webpack": "gulp watchView",
     "test": "npm-run-all -p test:*",
-    "test:unit": "mocha --exit -r ts-node/register test/pure-tests/**/*.ts",
+    "test:unit": "LANG=en-US mocha --exit -r ts-node/register -r test/mocha.setup.js test/pure-tests/**/*.ts",
     "test:view": "jest",
     "preintegration": "rm -rf ./out/vscode-tests && gulp",
     "integration": "node ./out/vscode-tests/run-integration-tests.js no-workspace,minimal-workspace",
diff --git a/extensions/ql-vscode/test/mocha.setup.js b/extensions/ql-vscode/test/mocha.setup.js
new file mode 100644
index 000000000..ec4b15613
--- /dev/null
+++ b/extensions/ql-vscode/test/mocha.setup.js
@@ -0,0 +1 @@
+process.env.TZ = 'UTC';
diff --git a/extensions/ql-vscode/test/pure-tests/date.test.ts b/extensions/ql-vscode/test/pure-tests/date.test.ts
index 2d00f2536..9dccbb500 100644
--- a/extensions/ql-vscode/test/pure-tests/date.test.ts
+++ b/extensions/ql-vscode/test/pure-tests/date.test.ts
@@ -5,7 +5,7 @@ import { formatDate } from '../../src/pure/date';
 
 describe('Date', () => {
   it('should return a formatted date', () => {
-    expect(formatDate(new Date(1663326904000))).to.eq('Sep 16, 1:15 PM');
-    expect(formatDate(new Date(1631783704000))).to.eq('Sep 16, 2021, 11:15 AM');
+    expect(formatDate(new Date(1663326904000))).to.eq('Sep 16, 11:15 AM');
+    expect(formatDate(new Date(1631783704000))).to.eq('Sep 16, 2021, 9:15 AM');
   });
 });