Old months keep their old rate — a revision never re-prices closed hisaab. A design number that does not exist yet is created the moment you rate it.
Pick a design above.
Accounts are managed in Settings.
Calls every read path and reports what threw. Reconciliation should read zero forever; the day it does not, it shows here first.
The account decides the firm — and so the TDS. An account is never deleted: switch it off and its history stands.