until now vs hitherto

until now and hitherto both are adverbs.

Word	Noun	Adjective	Verb	Adverb
until now	No	No	No	Yes
hitherto	No	No	No	Yes

As adverbs, until now and hitherto are synonyms defined as:

until now and hitherto: used in negative statement to describe a situation that has existed up to this point or up to the present time

Other synonyms of until now include as yet, heretofore, so far, thus far, til now, up to now, yet.

until now (adverb)	hitherto (adverb)
used in negative statement to describe a situation that has existed up to this point or up to the present time	used in negative statement to describe a situation that has existed up to this point or up to the present time