diff --git a/powerwall.sh b/powerwall.sh
index aec66096..e52c5e8b 100755
--- a/powerwall.sh
+++ b/powerwall.sh
@@ -143,7 +143,7 @@ CHECKACV_SERVICE(){
 				echo "($datetime) alert sent" >> ${logacv}
 				battstatus=0
 			else
-				errtime=`date +%s`-$(stat -c %Y ${logacv})
+				errtime=$(expr `date +%s` - $(stat -c %Y ${logacv}))
 				[ $errtime -ge 60 ] && echo "($datetime) normal voltage: ${INPUTACV}V" >> ${logacv}
 				if [ $voltstatus -eq 0 ]; then
 					echo -e "($datetime) Service Startup\nNormal voltage detected\nVOLTAGE: ${INPUTACV}V" | mail -s "Power Nominal" $email_alert