What is the difference using the de particle?

Question:

I know there's a difference but...

後で十二時間ぐらい日本にいます

and

後十二時間でぐらい日本にいます

Answer:

I think you need to tell us where you read those 2 sentences. In fact, it depends on what you try to say. So without knowing it, there is no accurate answer.

The 2nd sentence is not correct. So you can't use it anyway.