diff --git a/src/time.c b/src/time.c
index 9039a80..4d946fa 100644
--- a/src/time.c
+++ b/src/time.c
@@ -127,23 +127,25 @@
         }
     }
 
-    if (tm.tm_isdst < 0) {
-        zoneshift = NULL;
+    /* years cannot be negative */
+    if (tm.tm_year < -1900) {
+        ERRARG("time");
+        return NULL;
+    }
+
+    if (tm.tm_gmtoff == 0) {
+        /* time is Zulu (UTC) */
+        if (asprintf(&zoneshift, "Z") == -1) {
+            ERRMEM;
+            return NULL;
+        }
     } else {
-        if (tm.tm_gmtoff == 0) {
-            /* time is Zulu (UTC) */
-            if (asprintf(&zoneshift, "Z") == -1) {
-                ERRMEM;
-                return NULL;
-            }
-        } else {
-            zonediff = tm.tm_gmtoff;
-            zonediff_h = zonediff / 60 / 60;
-            zonediff_m = zonediff / 60 % 60;
-            if (asprintf(&zoneshift, "%s%02d:%02d", (zonediff < 0) ? "-" : "+", zonediff_h, zonediff_m) == -1) {
-                ERRMEM;
-                return NULL;
-            }
+        zonediff = tm.tm_gmtoff;
+        zonediff_h = zonediff / 60 / 60;
+        zonediff_m = zonediff / 60 % 60;
+        if (asprintf(&zoneshift, "%+02d:%02d", zonediff_h, zonediff_m) == -1) {
+            ERRMEM;
+            return NULL;
         }
     }
 
@@ -155,7 +157,7 @@
                         tm.tm_hour,
                         tm.tm_min,
                         tm.tm_sec,
-                        (zoneshift == NULL) ? "" : zoneshift);
+                        zoneshift);
     } else if (asprintf(&date, "%04d-%02d-%02dT%02d:%02d:%02d%s",
                         tm.tm_year + 1900,
                         tm.tm_mon + 1,
@@ -163,7 +165,7 @@
                         tm.tm_hour,
                         tm.tm_min,
                         tm.tm_sec,
-                        (zoneshift == NULL) ? "" : zoneshift) == -1) {
+                        zoneshift) == -1) {
         free(zoneshift);
         ERRMEM;
         return NULL;