diff --git a/TimeSeries.pm b/TimeSeries.pm
index 1206403..3759090 100644
--- a/TimeSeries.pm
+++ b/TimeSeries.pm
@@ -113,7 +113,8 @@ sub add_timestring {
     my $timestamp;
     if (defined ($zone)) {
 	# adjust for timezone
-	my ($zs, $zh, $zm) = $zone =~ /([+-])(\d\d)(\d\d)/;
+	my ($zs, $zh, $zm) = $zone =~ /([+-])(\d\d)(\d\d)?/;
+        $zm //= 0;
 	$min -= ($zs eq '-' ? -1 : +1) * ($zh * 60 + $zm);
 	$timestamp = timegm_nocheck($sec, $min, $hour, $day, $mon-1, $year);
     } else {