testing date
(A) The term testing date means the date used for determining the status of controlled group members as either component members or excluded members. That testing date is then also used to determine which taxable years of those component members are to be subjected to the controlled group rules. Generally, a member's testing date is the December 31st date included within that member's taxable year, whether such member is on a calendar or fiscal taxable year. However, if a component member of a controlled group has a short taxable year that does not include a December 31st date, then the last day of that short taxable year becomes that member's testing date.