It's years when I enjoyed myself so much.翻译,并解释这里when的用法

应该不能用when的.应该说：It's years since I enjoyed myself so much.我已经有多年没这么高兴过了.要用when,那it's后的名词就应该是某个时间点.如：It was the year of 1949 when the People's Republic of China ...我在一本练习册上看到的，一般不会出错的啊什么练习册？看错答案的吧？It was the year of 1949 when the People's Republic of China was founded.这是定语从句的。老师说这里when引导了一个表示时间的定语，怎么理解？那这里的years就应该加冠词，特指。It was the years when I enjoyed myself so much.那是那些年，我非常快乐。（其实这里的years尽管有几年，但还是时间点的）但years是时间段。（因为不知道是多少年）when引导的是定语从句，when在从句中指时间。