We've changed our rules for calculating eligibility date for certain benefits
from 1 year of service to 90 days of service. Yet even after 90 days,
enrollment cannot be until Jan 1, Apr 1, Jul 1, Oct 1 of any year. How can I
adjust the following formula to calculate 90 days plus the time to the next
interval above to arrive at an appropriate eligibility/enrollment date?

=IF(ISBLANK(D3),"not yet",DATE(YEAR(D3)+1,INT((MONTH(D3)+2)/3)*3+1,1))