diff --git a/time/time-lib.pl b/time/time-lib.pl
index 965067526..9ac320af8 100755
--- a/time/time-lib.pl
+++ b/time/time-lib.pl
@@ -244,7 +244,7 @@ return undef;
 
 sub number_to_weekday
 {
-return ucfirst($weekday_names[$_[0]]);
+return defined($_[0]) ? ucfirst($weekday_names[$_[0]]) : undef;
 }
 
 # Returns 1 if this system supports setting the hardware clock.