What functionality is missing? What is unsatisfying with the current situation?

I think in options where you can put in a date it would be easier to have a drop down for the calendar you have set up. One where you could type the date in a box, or pick a calendar and select a date based on the format youve chosen.  

How does this feature request address the current situation?

This would make it easier to quickly select dates, and could link things to a default timeline  

What are other uses for this feature request?

I think it opens options to add things to timelines (founding of a city or birth of a king) and have more than just events show up in timelines, so you dont have to have 3 different things for one character, you could just have their birth and death link to a default timeline and go into the timeline to add it to others as needed