testing period
(B) The term testing period means the time period used for determining the status of controlled group members as either component members or excluded members. The testing period begins on the first day of a member's taxable year and ends on the day before its testing date. (Generally, the testing date is December 31st, but for a component member having a short taxable year not ending on December 31st, the testing date for the short taxable year of that member (and only that member) becomes the last day of that member's short taxable year.) Thus, for a member on a fiscal taxable year, the portion of its taxable year beginning on December 31st and ending on the last day of its taxable year is not taken into account for determining its status as a component member or an excluded member.