Today is November 18th, 2017. If you want to refer exactly to the day October 18th, 2017, there are two options: 1) Use the exact day and say "October 18th" (this is more common) 2) Say "exactly one month ago" (also works but less often used) If you just say "one month ago", then to a natural speaker that means "approximately one month ago". If something happened more than 14 days ago, do not use days ago to approximate when it occurred. Hopefully that answered the question.