The general rule is that the payment must be credited as of the date it is received. 
The credit union may set reasonable requirements for payments, including a cut-off time. The cut-off time on the payment due date generally must be 5 p.m. or later. Payments received after the established cut-off time will generally be credited as of the next business day. If the credit union listed reasonable requirements for conforming payments on your billing statement, but accepted a payment you made that did not meet those requirements, the credit union must credit the nonconforming payment within five days of receipt.

If the credit union promoted electronic payment via their website, then generally any payments made via the credit union’s website prior to the specified cut-off time would be considered a timely payment.