It's hard to explain....

In fact, both "đó" and "kia" means "that" if it's used to refer to a place, a location in the position. You can used each of them. No problem ! No difference.

But in the time, there is no rule for them. You can't exchange between them. If not everyone will misunderstand your mind.

There are a lot of expressions:
- ngày đó / hôm đó = that day
- sau đó = after that (in the time)
- ngày kia / ngày mốt = the day after tomorrow
- ngày hôm kia = the day before yesterday
.........

After listing out these expressions, I suddenly think that it's better for you to understand "that" = "đó" in the time. The word "kia" is used quite rarely and usually combined with another words to be able to have the meaning.

hope that help