天荒地老纯属童言无忌
[tiān huāng dì lăo chún shŭ tóng yán wú jì]
'Tian huang di lao' is often used to describe an extremely long time. In this context, this phrase suggests that something timeless (or seemingly eternal) is actually innocent talk without any restraint. It conveys a romantic but ephemeral feeling of young love or wishful talk.