DTO with Dateparse OpenAPI/Java and Login in JSF with LoginDTOs, how to split front- and backend for safety?

Question

Here is my BookCustomerDTO(Data Transfer Object)
Is it also able to change the Date-Scheme, so in the Java JSF-Frontend there is a LocalDateTime and not a OffsetDateTime?

And here a part from the controller

My frontend looks like this where the problem occurs when i try to safe the date
And with this method i redirect to the datatable where the data is displayed. Is it ok if i iterate among all loginDTOs and check the username and password and if they match i create a session and redirect to the datapage.