time for doing something / period of time / season

important, significant

importance, consequence, significance

extraordinary, unusual

emergency

very; extremely / exceedingly; tremendously

history / the study of history

age, era, period (of time) / the times, spirit of the age / antiquity / a time in one's life, one's days (in college etc.)

the Mesolithic

(historical or contemporary) any of several kingdoms, empires, or states of the Chinese people