diff --git a/misskey-service.nix b/misskey-service.nix
index 44b573b..6c82888 100644
--- a/misskey-service.nix
+++ b/misskey-service.nix
@@ -173,7 +173,7 @@ in {
         ];
         text = ''
           now="$(date +%s)"
-          stamp="$(echo $now - 2592000 | bc)"
+          stamp="$(echo "$now - 2592000" | bc)"
 
           tmp="$(mktemp -d)"
           date="$(date --date="@$stamp" -I)"