When someone says "I am out of the office until 10/23", they are typically indicating that they will be returning to the office on 10/23. In this context, "until" means up to and including the specified date. Therefore, they are expected to be back at work on the mentioned date, which is 10/23 in this case.
If the person intended to return on 10/24, they would usually say something like "I will be out of the office until 10/23 and will return on 10/24." The use of "until" in the original statement implies that the absence will end on the specified date, which is 10/23.