[mykwyner]

Your friend is right. In this sentence, 1993 is functioning as a noun. It is the name of a year, therefore it is a proper noun.

I worked at Starbucks could mean that you worked inside a Starbucks coffee shop. I could also mean that you stood in front of a Starbucks coffee shop begging for money.

I worked for Starbucks means that you were employed by Starbucks. It could mean that you sold coffee at the counter, swept the floor, or piloted their corporate jet.