They had undoubtedly made progress in the six years that separated the twoperiods.
他们无疑在那两个时期之间的6年里取得了进步。
《柯林斯英汉双解大词典》
2
There were actually twoperiods of rain and lake formation: the first one began about 37,000 years ago; and the second one dates from about 10,000 years ago.