But the way people use it is not for exact dates and deltas. If today is March and I know something will happen in August, I'll say "this will happen in 5 months from now", but I've never heard anyone add days and weeks to such a duration. If today is 31st o'March and you tell me "next month", that could mean anything between one or thirty days, i.e. the difference between 31st o'March and 1st o'April is one month.

Edit: Also the same way that "tomorrow" can be in 5 minutes.