diff --git a/src/time/tzset.c b/src/time/tzset.c
index b96ea158..0cd47cf2 100644
--- a/src/time/tzset.c
+++ b/src/time/tzset.c
@@ -21,8 +21,8 @@ static char dst_name[TZNAME_MAX+1];
 
 /* all elements are zero-based */
 static struct rule {
-	char month;
-	char week;
+	signed char month;
+	signed char week;
 	short day;
 	int time;
 } __dst_start, __dst_end;