Very similar they both are; but there is thin line that separates the two words.
Sometime – as an adverb (not comparable) means at an unstated or indefinite time in the future.
✔ I'll see you at the pub sometime this evening. This will certainly happen sometime in the future.
sometime – (not comparable) as an adjective means:
Former, erstwhile; at some previous time.
✔ My sometime friend and mentor.
Synonyms include former, earlier, erstwhile, ex-, previous (occasional)
SOMETIMES – as an adverb (not comparable) means on certain occasions, or in certain circumstances, but not always.
✔ Sometimes I sit and think, but mostly I just sit.
✔ It is good that we sometimes be contradicted.
The English Dictionary. Extracted from wikitionary.org