(I don't understand how Chris is using the @ to indicate column names. Doesn't work for me. In my solution, each column is a named range (Days and Hours), and the bare range name appears in the formula.)
It's important to note here that both columns contain numbers. The Hours column contains a fraction; since it's formatted as a time, that's interpreted as a fraction of a 24-hour period. That has to be true, or the formula
wouldn't do anything useful, even for positive values.
Now, we're adding together to values that are always both positive or both negative. The Days value always has the correct sign, but the Hours value is always non-negative, even when it should be negative. So we need to change the Hours value when Days is negative: