A lot of our students, for example the ones from Italy because they use the same word for both, have some difficulty with the difference between within and by in time expressions. However, the difference is actually quite simple:
Use BY with a point in time
Use WITHIN with a period of time
Let’s look at the following examples…
The job application must be submitted by 31 January 2015.
Our electricity bill must be paid within 2 weeks.
As you can see ’31 January 2015′ is a point in time, whereas ‘2 weeks’ is a period. There is also another possibility to mention which is synonymous with within, and that is in…
Our electricity bill must be paid in 2 weeks.
There you go, nice and easy!