Currently a 'time' is ambiguous if it doesn't come with a timezone. And if it does come with a timezone, then there is no issue with that future shift. You wouldn't redefine EST you'd create an EST2 that was shifted (and which people would colloquially call 'Eastern' after long enough).
So the exact duration between any two times would be easily computed, so long as those times were precise enough to include their timezone-- which they have to be already.
So the exact duration between any two times would be easily computed, so long as those times were precise enough to include their timezone-- which they have to be already.