Semantically, how does 'before' differ from 'till'?

Question

till {prep. [here] conj., and adv.}

Etymology: [..] Probably originally a noun * til = Old English till fixed point, station [...]
hence the const. with genitive: prop. ‘with the limit or goal of (the place or time named)’. [...]
To the same root belong Old English til adjective ‘to the purpose, serviceable, good’ [...]

II. Of time.
[5] b. After a negative, denoting the continuance of the negative condition up to the time indicated (and implying its cessation then); thus nearly equivalent to before. Cf. B. 1b.

OED's use of the adverb nearly above, implies difference(s) between till and before. So what are these semantic drifts?
Please expose and explain them.

Etymologically, till means: TO some fixed point or station. So this 'TO' can be interpreted as approximation to the fixed point/station, without knowing whether the fixed point is attained. Which meanings of before, does this interpretation of till neglect?

Answer 1

I'll be here till 3.

I'm going to leave before 3.

I hope I understood your question.

Answer 2

Sometimes the two words are more or less interchangeable, but they do have subtle differences.

before

preposition

previous to; earlier or sooner than

Source: Dictionary.com

until

conjunction

up to the time that or when; till

Source: Dictionary.com