It was about noon when we arrived at the top of the mountain.这句少一个从属连词when引导时间状语从句.
如果主句稍改动一下,像下面这样就可以写成强调句
It was at about noon that we arrived at the top of the mountain.强调句子的时间状语.主句要有一个完整的介词短语做时间状语就可写成强调句.