langsyne vs good old days

langsyne and good old days both are nouns.

Word	Noun	Adjective	Verb	Adverb
langsyne	Yes	No	No	Yes
good old days	Yes	No	No	No

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

Other synonyms of langsyne include auld langsyne, lang syne, old times.

langsyne (noun)	good old days (noun)
past times remembered with nostalgia	past times remembered with nostalgia

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