`eventspan`s to be defined with precise dates

[Pamplemousse]

Given that event can be specified as exact dates, it would be nice to be able to bound by such dates as well.

Right now, for an eventspan defined between two precisely defined events, the tags for the events don't align with the begining and end of the eventspan.

[gnist-dev]

That should be a really quick fix to do. I'll get around to it.