Grammar

You don't really need to write more then one thread on this SAME topic.

Your two given sentences don't quite mean the same thing. Though their meanings somewhat overlap a bit.

First and foremost before I go into the by and before parts , your use of the verb reach makes little sense in either sentence. Did you mean to use arrive there or get there instead?


The person promises to get there at 9:00PM or slightly earlier if the situation permits.

The person promises to get there no later the 8:59PM.

tsarstepan,
sorry, it is because I couldn't see the threads after posting and I am not sure why. I think I have understood the difference between 'before' and 'by'.
'Before 9Pm' means at some time earlier than 9pm. 'By 9pm' means at or before.

tsarstepan,
thanks a lot for your examples.