We use "by" to specify time or date, as in
I have to go by 9 am.
We can also use "at" here. When to use "by" / "on" / "at"?
When "by" should be preferred over "before" or "till"?
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So,
To answer your follow-up question: Use before when you have to leave earlier than 9 am. Till is the informal version of until. Until has many uses, just like the on/at/by, but I think the main use here is
So "I have to leave until 9 am" doesn't really make sense. One might interpret this as you have to be gone or absent until 9 am. But as you can see, it has a completely different meaning than with the words at or by. You can say
This means you have to stay. And when the time is 9 am, you are free to leave. Depending on the context, it might imply that you will leave at exactly 9 am. |
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"By 9 am" means any time before 9 am. "At 9 am" means at 9 am specifically. We don't use "on" with time of day, but we do use it with days: "On Monday" means sometime during Monday. "By Monday" means any time before Monday. |
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Use by when some action or event needs to be completed before a specific date or time, but it is possible that it could happen earlier.
(allowing the possibility of leaving earlier)
(allowing the possibility of completing it earlier) When we have a specific date or time use on for a date, use at for a time.
we can use both on and at in the same sentence:
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