Cute dress!
I'm no expert, but this looks 90's to me, though I can see why you thought 60's with the length. Are there any labels? I see 90's because of the slip styling, which was massive then due to the influence of Courtney Love and other ladies in the grunge movement wearing vintage lingerie/slips as dresses. Usually worn with Doc Martens or chunky mary janes. One of my favorite 90's fast fashion labels is All That Jazz, and they made a lot of dresses similar to this one.
I'm curious to see what the experts think!