lang syne vs auld langsyne

lang syne and auld langsyne both are nouns.

Word	Noun	Adjective	Verb	Adverb
lang syne	Yes	No	No	Yes
auld langsyne	Yes	No	No	No

As nouns, lang syne and auld langsyne are synonyms defined as:

Other synonyms of lang syne include good old days, langsyne, old times.

lang syne (noun)	auld langsyne (noun)
past times remembered with nostalgia	past times remembered with nostalgia

lang syne (adverb)	auld langsyne (adverb)
of the distant or comparatively distant past