good old days vs lang syne

good old days and lang syne both are nouns.

Word	Noun	Adjective	Verb	Adverb
good old days	Yes	No	No	No
lang syne	Yes	No	No	Yes

As nouns, good old days and lang syne are synonyms defined as:

Other synonyms of good old days include auld langsyne, langsyne, old times.

good old days (noun)	lang syne (noun)
past times remembered with nostalgia	past times remembered with nostalgia

good old days (adverb)	lang syne (adverb)
	of the distant or comparatively distant past