1. something established (or formed) in a certain time, must use 于. for example, 公司成立于9月。 2. someone established (or formed) something in a certain time, then you can both use 于 and 在. for example, 我们在9月成立了公司。/　我们于9月成立了公司。

在is a preposition which usually comes before location noun，time noun，it is more used in oral language，于 is more formal ，it is not natural and common to use it before location noun。

于 is more formal. you can see it in some advertisements or commercials. also, it is used in ancient Chinese texts, we seldom use it in our daily conversation. 在 is more used in oral language. we use it everyday :) they both mean the same thing, though.

于是介词，没有实际意思，在许多地方与在的用法一样，如：我生在1994年，或，我生于1994年。 在可以做动词，介词，副词，名词，助动词，如：我在家。我生在1994年。在建造。等等。 i wish it is uesfulness for you.