1、dating back to childhood在句子中做定语,修饰“their friendship"
类似的还有：Their cooperation built on the basis of faith will be successful.这项建立在信任基础上的合作会取得成功.
2、when properly done在句子中做状语
类似的如：The teacher,when recovered from the illness,will come back to school as soon as possible.当这位老师从疾病中恢复以后,会尽快回到学校.