The context tells you whether or not N hours ago is precise. I would say that, typically, it is not. To be clear, you could add about to the expression:
about N hours ago.
Otherwise, the context will help you determine that it is precise, or you might add exactly to the expression:
exactly N hours ago.
I don't think your usage of N hours ago is correct. For example, if it's 8:40, and you say something with one hour ago, then I don't think it is perceived as anything between 7:00 and 7:59. Instead, it is understood as 7:40 plus or minus a few minutes. In other words, the approximate interval is centered around 7:40 and it is not the interval 7:00 to 7:59.
As for your particular examples, I don't think there's a rule. You are just going to have to choose what feels best.
If you really want to use some rule, you could round to the closest hour. So if something happened 1 hour and 20 minutes ago, you would round down to 1 hour and say X happened 1 hour ago. So for your first example, you could say 1 hour ago, and for the second, you could say 2 hours ago.
Including something like about, close to, almost, etc also sounds natural. Using your numbered examples:
- I got here about an hour ago.
- I bought this almost two hours ago.
This kind of question is asked frequently. English is not that precise about time. As stangdon points out, "the last two years" could mean the past 730 days, or it could mean last year and the year before. You have to determine from context.
I've been in bed sick the last three days (yesterday, the day before, and the day before that, plus possibly today).
I've spend the last day in bed (the previous 24 hours).
In general it's not important to be that exact. You can consider these rough estimates. However, because English is not precise with these expressions, we will usually add additional information if precision is important, or you should ask for more details:
If you are still sick after three days -- I mean by Monday morning -- then call me or go to the emergency room.
Doctor, you said I should take two of these pills for the next six days. Does that mean I should also take two pills today?
Best Answer
There are actually 3 phrases: “let it go”, “let it all go”, and “let all go”. “Let it go” usually refers to a thing that has already been mentioned, which should be let go. (Or this song from the movie Frozen.) “Let it all go” means that you should let everything go, or that there are multiple things (that have already been mentioned) that should be let go. “Let all go” is sometimes the same as the former (first) “let it all go”, but it more often means that you should unwind and let yourself go. “Let all go” is also slightly less formal in some cases.
Sources: Oxford Dictionaries