Which date attributes do you want to be able to filter? There's join date, last online date, Suspended until. There are "virtual" attributes that Flarum currently doesn't compute like last post, last discussion. Some attributes that won't be available without additional logic and storage like last login, last password reset, last email change. And possibly more added by other extensions.
If we're going to open this to bounties I think this should definitely be 2 separate proposals.
There is probably zero requirement for the export feature to be part of FoF User Directory, it can be its own extension.
Some users might think the bounty would be better spent adding the filter feature to Flarum's admin panel user list rather than a community extension. I think this needs to be decided before creating a bounty because if there ends up being 2 different bounties for implementing it in 2 different places, this might cause less funds to be available for each one as sponsors will only pick one.
