The example "I worked until 2am last night" implies that you worked up to and including 2am whereas "I have to leave by 9pm" means that by 9pm you will have left already.
What Is The Difference Between By And Until?
While both 'by' and 'until' are used to indicate actions in time, there are essential difference between the two usages. The misuse or wrong use of these two words is one of the most common grammatical errors you will encounter, along with other such mistakes as 'accept' instead of 'except', 'all ready' instead of 'already'.
The word 'until' is used to denote a usage that connotes an action or event any time before the time period that usually follows the word, but not beyond that time. The word 'until' is used to indicate the time period that a situation or event has been going on for. For example - I worked until 2 a.m. last night. It is also used to denote negativity. For example - There will be no voting until day after tomorrow.
In contrast, the word 'by' is used to denote an event or a situation that happens at or before a given time. For example - You have to close your shop by 9 p.m. The word 'by' is used also in an interrogative sense. For example - Will the product be ready in time for the launch?
The word 'until' is used to denote a usage that connotes an action or event any time before the time period that usually follows the word, but not beyond that time. The word 'until' is used to indicate the time period that a situation or event has been going on for. For example - I worked until 2 a.m. last night. It is also used to denote negativity. For example - There will be no voting until day after tomorrow.
In contrast, the word 'by' is used to denote an event or a situation that happens at or before a given time. For example - You have to close your shop by 9 p.m. The word 'by' is used also in an interrogative sense. For example - Will the product be ready in time for the launch?