I have given myself until the end of September to finish the book.
Until is the right solution here because you are talking about a certain periode of time. It stresses the stretch of time from now till the end of September. You have time until September to finish the book
But you can also write the same sentence with "by" and it will be something like; By the end of September I will have finished the book. With the last sentence you stress a specific point in time "end of September".
few example sentences with "until":
- Let's start now and work until dark
- I work from 9 until 5 o' clock.
- We will stay here until it stops raining
- Go on until you get to the level crossing
Few examples with "by"
- The train leaves at 6.10 so you better be at the station by 6 o'clock
- By the end of June I will have read all those books
Note that "by" indicates at that time or before but not later ! It's getting used for time/dates/periodes.
I did my English exam last month and that were one of the things we had to know To my mind, it isn't that hard. But there is a big difference to know something and using it during a conversation. We have to practise it as much as possible to get it in to our system
But it is so nice !!! And fauling up every now and then is't that bad, we are just human after all