In my area (in the UK) we regularly use 'for' as a preposition of time. For example, "Could you finish this for 5?" (as in, "Could you finish this by 5?"), a non-native English speaker corrected me and said 'for' can't be used as a preposition of time. I have tried to research this, but it seems like it's something that isn't technically correct. So, I'm wondering if this is just a dialect quirk of where I live, if it's a British English thing, or if this is actually correct but isn't widely taught. If anyone had any solid evidence either way, that'd be great, as I'm really interested. 

Original Post

Hello, Lauraaliali, and welcome to the Grammar Exchange.

I don't think the difference between "for" and "by" in adverbials of time indicating deadlines is a BrE/AmE issue.

Both are correct but slightly different. While "for" introduces the due date on which some task needs to be completed, "by" means "no later than":

- Could you finish this for 5? ( Delivery is expected to take place at 5.)

- Could you finish this by 5? (Delivery is expected to take place anytime no later than 5, or at 5 at the latest.)

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