What's the difference between "IN an hour" and "an hour LATER"?

ornate clock on tower
Time can be a tricky topic in English

A lot of people use in and later incorrectly. Are you one of them? You are if you've said something like this:

A: When are your going to Hawaii?
B: I’m going two days later. [Incorrect]

The problem is that if you are talking about an amount of time from now, you must use "in":

B: I’m going in two days. [Correct]

So then how do we use "later"? "Later" is a comparing word, similar to "bigger", "smaller", "shorter", "longer", etc. So to compare, we need at least two points in time, like Tuesday and three days after that:

B: My partner is leaving on Tuesday, and then I’m leaving three days later.

To be strictly correct, you can add “later than him,” but most communication doesn’t need this. The listener already understands.

There is a second meaning for later, and that’s in the future at an unspecified time. This is a very popular meaning, and you’ve probably used this before:

A: Are you going to eat lunch now?
B: No, I’m not hungry. I’ll eat later.
A: Oh, I’m starving. I’m going for lunch now.
B: Okay, see you later.

If you are one of the many people who have been making a mistake with this language, it will be tough to change your speaking habits. But, with daily practice, you’ll be able to do it in no time.