How was Hillel II allowed to change the Jewish calendar?

Declaring the new month by observation of the new moon, and the new year by the arrival of spring, can only be done by the Sanhedrin. In the time of Hillel II, the last President of the Sanhedrin, the Romans prohibited this practice. Hillel II was therefore forced to institute his fixed calendar, thus in effect giving the Sanhedrin's advance approval to the calendars of all future years. Until Hillel II's time, the calendar varied irregularly because it depended on the testimony of witnesses who had seen the new moon, and this didn't always happen on the first possible night. When Hillel II fixed the calendar, there was no reason not to do it so as to prevent the holidays from coming at inconvenient times; the fluctuations of the fixed calendar don't exceed the variations in the calendar when it depended on witnesses. Ask a follow-up question