You can pass the formatted attribute as a node:
object @invitations
attributes :email, :active
node(:formatted_invitation_sent_at){|invite| invite.invitation_sent_at.strftime(some_format)}
node(:formatted_invitation_accepted_at){|invite| invite.invitation_accepted_at.strftime(some_format)}
Alternatively, make this a helper_method in your model, and pass that helper_method instead of the attribute.