Expressions – ‘By the Time’ vs ‘The Moment’: Expressions of Time

"X will be there until the 11th." When will X leave? On the 12th, right?

X would leave on the 11th.

"X won't be there until the 11th." When will X be there? On the 11th, right?

Yes, they would arrive on the 11th.

"X hasn't done it until high school" In this case X did it at some time during high school, right?

Yes, it means that X did something for the first time when they were in high school.

"X has been doing it until high school" In this case it seems to be unclear when X stopped. During high school or before starting high school.

To me, it clearly means that X stopped doing something when they started high school.

Then with an attempt of clarification:

"X has been doing it until he entered high school" Does it mean X stopped doing it before or just after he entered high school? Or could it be both?

It means that X stopped doing something when he entered high school. Writing it this way is a possibility but, not really necessary. It was expressed sufficiently with the previous quote.

Is this clear to native speakers or is it just me?

That is clear and sentences that use the word until, like that, are used regularly by native speakers.

American English most common speech usage examples using 6 o'clock:

• 6:00 we would normally just say "Six". If we really want to point out the exact time we might say "Six on the dot".

• 6:01-6:29 normally uses after. Past is fine here too but not used as much. Also if you knew the hour, then you would probably just say the time. Example - "It is 6:25" or if you know it is 6-something you say "25 after". The one anomaly is if you use "quarter" and "quarter past" is just as common or more as "quarter after".

• 6:30 we say, "half past". Past is used almost exclusively. If you said "half after" it sounds a little weird but still OK. Example - "It is 6:30" or if you know it is 6-something you say "half past".

• 6:31-6:59 would normally use till. After and past could be used and understood. They just normally aren't - "40 minutes after" isn't used much. Example - "It is 6:40" or if you know it is heading towards 7 then you would say "20 till".

Again you could say "20 till 7" or "quarter after 3" or whatever but if we are saying the whole time we just normally say it. So "10 minutes till 11" is usually just said "ten fifty".

As stated by some comments the word "to" is also said instead of "till". It may even be more commonly written. Using "to" would be very acceptable in speech but I feel the word "till" is more common in America.