diff --git a/check_systemd_service.sh b/check_systemd_service.sh
index 9cb4434..ab69f90 100755
--- a/check_systemd_service.sh
+++ b/check_systemd_service.sh
@@ -68,8 +68,8 @@ fi
 start_time=$(systemctl show "$UNIT_NAME" --property=ExecMainStartTimestamp --value)
 end_time=$(systemctl show "$UNIT_NAME" --property=ExecMainExitTimestamp --value)
 current_time=$(date +%s)
-start_time_diff="undefined"
-end_time_diff="undefined"
+start_time_diff_perf=""
+end_time_diff_perf=""
 
 if [ -n "$start_time" ] && [ "$start_time" != "n/a" ]; then
   start_time_diff=$((current_time - $(date -d "$start_time" +%s)))