Like so many other data items in PeopleSoft, an employee's dependent's data is also effective dated. Something to keep in mind with the effective date on a dependent, as it is maintained in the Update Dependent/Beneficiary screen, is that the effective date does not have to match whatever the enrollment date is for that employee's benefits. If the effective date on the dependent and the effective date on the enrollment match, there may be issues with enrolling the dependent into benefits.
One suggestion is to use the employee's hire date as the effective date. The enrollment date in benefits will always be later than the hire date, so it is a good way to avoid enrollment issues.