def METHOD_NAME(aws_accounts):
    existing_statement_list = [
        {
            "effect": "Allow",
            "sid": "ExpireStatement",
            "expires_at": "2023-01-24",
        }
    ]
    existing_document = ManagedPolicyDocument(
        policy_name="foo", version="bar", statement=existing_statement_list
    )
    new_statement_list = {"effect": "Allow", "sid": "ExpireStatement"}
    new_document = ManagedPolicyDocument(
        policy_name="foo", version="bar", statement=new_statement_list
    )
    merged_document: ManagedPolicyDocument = merge_model(
        new_document, existing_document, aws_accounts
    )
    assert (
        merged_document.statement.expires_at
        == existing_document.statement[0].expires_at
    )