diff --git a/scripts/backup.sh b/scripts/backup.sh
index da02955..f2ca29f 100755
--- a/scripts/backup.sh
+++ b/scripts/backup.sh
@@ -44,14 +44,14 @@ if [ "$1" = 'zammad-backup' ]; then
 
   while true; do
     NOW_TIMESTAMP=$(date +%s)
-    TOMORROW_DATE=$(date -d@"$(( $NOW_TIMESTAMP + 24*60*60 ))" +%Y-%m-%d)
+    TOMORROW_DATE=$(date -d@"$((NOW_TIMESTAMP + 24*60*60))" +%Y-%m-%d)
     
     zammad_backup
 
     NEXT_TIMESTAMP=$(date -d "$TOMORROW_DATE $BACKUP_TIME" +%s)
     NOW_TIMESTAMP=$(date +%s)
 
-    sleep $(( $NEXT_TIMESTAMP - $NOW_TIMESTAMP ))
+    sleep $((NEXT_TIMESTAMP - NOW_TIMESTAMP))
   done
 fi