If you've added StDate and EndDate to Client table, then it sounds like you are storing multiple records in Client table for each client. So then what are you storing in your transaction table? Sounds like you don't need 2 tables?
More typically, you wouldn't change the Client table at all, leaving your PKey as ClientID. Then you would add a transaction table that had all the same fields as Client table, plus a StartDate field. This transaction table would have a PKey of ClientID, StartDate; and a FKey relationship of ClientID mapping to Client table.
Beware of using datetime values as keys or unique constraints. It might work for low transaction systems, but datetime values are only accurate to .003 seconds. If you have the potential of 2 users updating the same record simultaneously, you may end up with unique key constraint errors.