Post dating check
before the advent of automated payments such as Direct Debits and standing orders.
If the bank does not spot that the cheque has been post-dated, the cheque would then probably be paid before you intended or returned unpaid if you have insufficient funds in your account.
Many banks state in their terms and conditions that post-dated cheques should not be written while some include a note at the front of chequebooks, advising that post-dated cheques should not be written.
So the safest practice is not to write post-dated cheques at all and set up a series of standing order payments, (provided the recipient is set up to accept automated payments to their account).
If the payer’s account is closed, the paying bank would not be obliged to honour the cheque and you would need to ask for payment by another means.
However the cheque would be evidence of the debt in the event of a dispute.
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But even under the alternative, why wouldn't a financial instrument (a check) NOT be legal? It basically becomes a promise to pay at some time, not on demand now.