We had a date for the marriage? And at least the "first time around", didn't it happen in 1795? A lot of things shifted to 1796 later on in DS, I know...
This episode, I believe, originally aired 1/2/68, so everything that happens from this point on is 1796. They should have done this when the episodes originally aired, but apparently they didnt realize the mistake of everything happening in 1795 until after the flashback ended.