diff --git a/TimeSeries.pm b/TimeSeries.pm
index e8e4976..2933fcf 100644
--- a/TimeSeries.pm
+++ b/TimeSeries.pm
@@ -598,7 +598,7 @@ sub get_ticks {
 	# 30 ... 90 days: 1 tick/week.
 
 	my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($firsttime);
-	$firsttime -= 86400 * $wday;
+        $firsttime -= 86400 * (($wday + 6) % 7);
 
 	($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($firsttime);
 	$sec = $min = $hour = 0;