diff --git a/check_systemd_service.sh b/check_systemd_service.sh
index 8147906..9cb4434 100755
--- a/check_systemd_service.sh
+++ b/check_systemd_service.sh
@@ -71,7 +71,7 @@ current_time=$(date +%s)
 start_time_diff="undefined"
 end_time_diff="undefined"
 
-if [ -n "$start_time" ]; then
+if [ -n "$start_time" ] && [ "$start_time" != "n/a" ]; then
   start_time_diff=$((current_time - $(date -d "$start_time" +%s)))
   start_time_diff_perf="| uptime=${start_time_diff}s"
 fi