To modify the end date of a Timeline, you must first “reclick” the start date, then click the new end date you want. This is tedious and super error prone. Say you wanted to push out the end date by 5 days, someone might intuitively look near the end date on the calendar and click on 5 days after the end date; but this would actually set the new start date, and the end date would be the start date plus the duration.
I’ve heard that the timeline used to visually have an arrow near the left side for Start Date and near the right side for End Date, that sounds much better than how it is now.
Example, the timeline is March 7th to 9th. If you just click the 14th, it sets the start date to 14th. To get the intended effect you would have to first click the 7th which is already selected, then click the 14th.