diff --git a/configure.ac b/configure.ac
index 5f201c2..c5c6448 100644
--- a/configure.ac
+++ b/configure.ac
@@ -61,6 +61,14 @@ AM_COND_IF([HAVE_DOXYGEN],
     [AC_CONFIG_FILES([Doxyfile])],
     [AC_MSG_WARN(["Doxygen not found; continuing without doxygen support"])])
 
+# Check for the presence of tm_gmtoff in struct tm. If we have this, then we
+# can use it to determine the true GMT offset
+AC_CHECK_MEMBERS([struct tm.tm_gmtoff],,
+    [AC_MSG_WARN(["Cannot find tm_gmtoff in struct tm, using slower method"])],
+    [#define _GNU_SOURCE
+     #include <time.h>
+    ])
+
 # Configuration headers
 AC_CONFIG_HEADERS([config.h])
 
diff --git a/lib/libx52/x52_date_time.c b/lib/libx52/x52_date_time.c
index bd8ad41..fb732aa 100644
--- a/lib/libx52/x52_date_time.c
+++ b/lib/libx52/x52_date_time.c
@@ -18,6 +18,33 @@
 #include "x52_commands.h"
 #include "x52_common.h"
 
+#if !HAVE_STRUCT_TM_TM_GMTOFF
+/* Slow method to compute GMT offset */
+static int _x52_compute_gmtoff(struct tm *timeval)
+{
+    char buf[20];
+    size_t written;
+    int offset;
+
+    /* This function uses strftime to get the timezone as a string, then
+     * converts that back into an integer offset.
+     */
+    written = strftime(buf, sizeof(buf), "%z", timeval);
+    if (written != 5) {
+        /* Expect %z to return 5 characters - e.g. +0530, -0700, +0000 */
+        return 0;
+    }
+
+    offset = 600 * (buf[1] - '0') + 60 * (buf[2] - '0') + 10 * (buf[3] - '0') + (buf[4] - '0');
+    if (buf[0] == '-') {
+        offset = -offset;
+    } else if (buf[0] != '+') {
+        offset = 0;
+    }
+    return offset;
+}
+#endif
+
 int libx52_set_clock(libx52_device *x52, time_t time, int local)
 {
     struct tm timeval;
@@ -36,10 +63,14 @@ int libx52_set_clock(libx52_device *x52, time_t time, int local)
 
     if (local) {
         ptr = localtime_r(&time, &timeval);
-        /* timezone from time.h presents the offset in seconds west of GMT.
-         * Negate and divide by 60 to get the offset in minutes east of GMT.
+        #if HAVE_STRUCT_TM_TM_GMTOFF
+        /* If valid, then timeval.tm_gmtoff contains the offset in seconds
+         * east of GMT. Divide by 60 to get the offset in minutes east of GMT.
          */
-        local_tz = (int)(-timezone / 60);
+        local_tz = (int)(timeval.tm_gmtoff / 60);
+        #else
+        local_tz = _x52_compute_gmtoff(&timeval);
+        #endif
     } else {
         ptr = gmtime_r(&time, &timeval);
         /* No offset from GMT */
diff --git a/tests/libx52/test_timezone b/tests/libx52/test_timezone
index 0a071c9..083e350 100755
--- a/tests/libx52/test_timezone
+++ b/tests/libx52/test_timezone
@@ -33,9 +33,6 @@ timezone_test()
         # Pacific Daylight Time
         # Default offset for clocks 2 & 3 should be +420 minutes
         off=420
-
-        # TODO: Remove the following line once #20 is fixed.
-        TEST_ID="$TEST_ID # TODO Fix issue #20"
     else
         # Pacific Standard time
         # Default offset for clocks 2 & 3 should be +480 minutes