Prepositions of time (and all types of prepositions) can be one of the hardest parts of English to use correctly because the rules are often quite difficult, and like most rules for a language, there are lots of exceptions.
In Unit 1 we looked at the prepositions of time ‘in’, ‘on’ ‘at’.
In this lesson we will look at the prepositions of time ‘within’ and ‘before’.
Prepositions of time – within
WITHIN: We try to answer all emails within 24 hours.
‘Within’ is commonly used to express that something will be done inside or not later than the period of time stated.
Note: time given must be an amount of time, NOT a specific time in the future.
For example:
We try to answer all emails within 24 hours.
We try to answer all emails within the following day. 
Other uses could be: within the next few minutes, within the next week, within the next six months, within this financial year etc.
Prepositions of time – before
BEFORE: The repairs will be completed before Friday.
Before is also used to express that something will be done inside or not later than the time stated.
Note: the time given must be a specific future time. For example:
The repairs will be completed before Friday.
We try to answer all emails before 24 hours.
Other uses could be: before 1pm, before next week, before July, before the start of the next financial year etc.