prepositions - When to use 'by' instead of 'at' or 'on' to specify time/date?

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"?

Answer

1. by
   not later than; at or before:



2. on
   use on to designate days and dates


3. at
   use at to designate specific times.

So,

1. 
   

   I have to go by 9 am
   
   means the latest you can stay is 9 am. You have to leave at or before 9 am.

   
   


2. 
   

   I have to go on 9 am
   does not make sense because on is used with days and dates.

   
   


3. 
   

   I have to go at 9 am
   means you have to leave when the time is (exactly) 9 am.

   
   

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To answer your follow-up question:

Use before when you have to leave earlier than 9 am.
Use by when you can leave specifically at 9 am or earlier.

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

until
use until to talk about something that will keep going on for a duration of time from a specific time to another.

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

I have to stay until 9 am.

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.