better handle start/enddates

[ut]

by now a post disappears the day after an event.

todo: calc this with enddate. if not set, stick with the day after, until it disappears

[ut]

additionally: "nightly" dates are written like "Fr 09.03.18, 21:00 ‒ Sa 10.03.18, 02:00 ", better "Fr 09.03.18, 21:00 ‒ 02:00"