The tail end of 1795... Potential Spoilers Ahoy!

they took great liberties with historical accuracy throughout the story.

 With Dark Shadows and the gothic genre in general, the setting of the story is isolated and exists somewhat outside of real-world events.  There's a link to the real world, but as we know, in Collinsport strange things tend to happen.   In this type of world, given the right circumstances, a witch trial could conceivably happen today even if it isn't happening anywhere else.  There's often a sense of the past invading the present in the gothic, and DS was spot on with capturing that type of thing.  Of course, it is inconceivable that we'd see a witch trial on a "real"-type show like Ally MacBeal (although that'd be pretty darn funny), but on Dark Shadows, it could happen.