Hi Christina, Do you have that field synced to Marketo? If so, it's as simple as a Smart Campaign using the "Data Value Changed" trigger, Account Creation Data field "Is not empty." The flow would start with a one year waitstep and then sent the email.
You may want to include a fliter in the flow step like "if lead status is client" just as a safeguard in case you have anyone that jumps ship early, I don't know your business so I don't know that it would be necessary.
Finally, you would likely want to create a report about the email's activity to be sent to you monthly (or weekly depending on volume) so you can keep up with when these are going out.
Hope that helps!