Hey @networkfactory 
As @JCorrell mentioned, you can absolutely have a timeline’s start and end date on the same day by simply double-clicking on the desired date:
As for The Workload Widget, you are required to choose only one timeline or date column per board:
If you would like to take other dates and timelines into account, you will need to either create multiple widgets or make a “master timeline” on your boards that is used to encompass the length of the entire project.
I hope this helps!
Cheers,
Jenna