Borgbackup: Don't schedule check-repo on day 0, that day doesn't exist.

Refs #198

morph/lib/borgbackup.nix

@@ -72,8 +72,9 @@ in {
 
     # Check repo once a month
     systemd.services.borgbackup-check-repo = {
-      # Once a month, 3h after last backup started
-      startAt = "*-*-" + toString backupDelay + " 18:33:22 UTC";
+      # Once a month, 3h after last backup started.
+      # Add "1" because day 0 is invalid.
+      startAt = "*-*-" + toString (backupDelay + 1) + " 18:33:22 UTC";
       path = [ pkgs.borgbackup ];
       environment = {
         BORG_PASSCOMMAND = "cat /run/keys/borgbackup/passphrase";