百度不到的旧时光
[băi dù bù dào de jiù shí guāng]
‘Baidu’ is a leading search engine in China; hence, 百度不到 (unreachable by Baidu) implies unreachable/irretrievable old times. So, this phrase conveys the idea of cherishing unattainable nostalgic memories or longing for past moments not easily retrieved or recalled by today's standards or searches.