从现在的自定时间变为只能在规定的4个时间段可考：
Exam periods will be:
1. 1st window: February 24 – March 14
2. 2nd window: May 26 – June 13
3. 3rd window: August 18 – September 5
4. 4th window: November 24 – December 19
望周知！