Well, there's an alternate opening where it's given a more concrete 18th century setting-- you can see it here.
[link] It's hard to tell exactly what decade it's supposed to be, but it seems mostly late 1770s to the early 1780s, judging by the gowns and hairstyles.
They got away from that in the final movie, and Belle's dress looks mostly 1850s-ish. But I figured it would be truest to her character to keep to the original concept and keep her late 18th century.