The CEPD has
forever: sadātanāya
but I've never heard that word before, nor can I find it in the tipitaka or commentaries.
sadā itself means "at all times" or "for all time" or simply "always". sabbadā is the same word.
nirantara means "always", but more in the sense of "without (nir) intermission (antara)".
I think the best word is sassatisama, which is an adverb explained as "sassatīhi samāna", meaning "in the way of the eternal". Found in the sentence:
"so nicco dhuvo sassato avipariṇāmadhammo sassatisamaṃ tatheva ṭhassati."
"It is permanent, constant, eternal, not subject to change; it stays that way forever (sassatisamaṃ)."
(DN 1 brahmajālasuttaṃ)
The Sanskrit equivalent is apparently "śaśvatīḥ samāḥ".
