around the time when

牛津英语必修二课文第二页第二句话This incident has received great interest due to reports of strange lights in the sky and of alien visits around the time the boy disappeared.
around the time the boy disappeared这句话是一句定语从句,可是为什么没有关系副词when呢?

around the time 和when在这句话中的语法作用和意思都是一样的
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