diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 1021d05..66b8876 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -277,17 +277,17 @@
     let tz = $TZ
   endif
 
-  " Force EST and then UTC, save the current hour (24-hour clock) for each
-  let $TZ = 'EST' | let est = strftime('%H')
-  let $TZ = 'UTC' | let utc = strftime('%H')
+  " Force different time zones, save the current hour (24-hour clock) for each
+  let $TZ = 'GMT+1' | let one = strftime('%H')
+  let $TZ = 'GMT+2' | let two = strftime('%H')
 
   " Those hours should be two bytes long, and should not be the same; if they
   " are, a tzset(3) call may have failed somewhere
-  call assert_equal(strlen(est), 2)
-  call assert_equal(strlen(utc), 2)
+  call assert_equal(strlen(one), 2)
+  call assert_equal(strlen(two), 2)
   " TODO: this fails on MS-Windows
   if has('unix')
-    call assert_notequal(est, utc)
+    call assert_notequal(one, two)
   endif
 
   " If we cached a timezone value, put it back, otherwise clear it