Learn English – The ambiguous “until”

time

As a non-native English speaker, It seems to me that the word "until" is quite ambiguous. It's been told that when it's used with a date it includes the date. (Does "until [date]" mean "before that date"?)

But it's still unclear to me.

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

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

If so, can anyone explain why it differs? Does it depend on whether it's negative and positive?

And how about other periods of time, like high school.

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

"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.

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?

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

Edit:
Seems like a lot of my confusion was made by this answer.
http://answers.yahoo.com/question/index?qid=20130205142054AAIqP06

Best Answer

"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.