关于by which和by which time

关于by which和by which time
请详细分析一下这两个非限制性定语从句引导词,何时使用哪一种?并烦请配上例句说明.(用during which和during which time也可以)

给你几个例句看下（来源google)
1.I don't have to worry about the old age during which I even can't take care of ...
2.The average length of labor during which these women received their meperidine was under 12 hours.
3.US Congress:HR 1347,To extend the period during which members of the Armed Forces deployed in contingency operations may request and ...
4.Socially Awkward Situations During Which It Would Be Acceptable to Mess With Texas.
5.Fish evolve a longer lifespan by evolving a longer period during which they reproduce.
1.In this post,he share his experiences during the past 5 years at learndirect,during which time they have strategically adopted a range of open source ...
2.Entered the service a fourth time some time in the year 1781,as a volunteer under Colonel William Moore and served about one month during which time he was ...
3.They lay there for three weeks during which time they made friends with the ...
4.Have been fostering for five years,during which time they have fostered 20 children.
5....registry will occur on 9/24/2006 from 9 PM to 9:45 PM EDT,during which time you will be unable to register,renew or manage .
这些都是google上搜到的原版英文句子,可以看出一个规律,during which的中心语大部分是表示时间段的名字例如period等；而duiring which time的中心语常常是具体的时间如5 years等.
具体的原理我不记得了,希望这样的对比能对你有所启发.