the good old days; halcyon days
A nostalgic set phrase referring to an idealized past time, often used in reminiscing or describing a bygone era.
My grandparents often tell me stories about the good old days.
This movie depicts the halcyon days of America.
懐かしい is an adjective meaning 'nostalgic' or 'dear', while 古き良き時代 is a noun phrase referring specifically to a past era seen as good.
A set phrase combining the classical adjective 古き (old) and 良き (good) with 時代 (era). The exact origin is uncertain, but it reflects a common nostalgic sentiment.