Well, a couple of things. First, the if you look at the shapesheet entries, the values are NOT date formatted. They are calculated values, buried in the supporting code. So, you cannot enter literal dates.
2nd, when you drop a timeline, there should be a popup menu asking for begin / end dates. This is the place to enter such info. You can always right click on the timeline to access this menu and change dates if desired.
Finally, there is the expanded timeline shape which will place control points on the main timeline and allow you to drag to desired start/end dates.
Wapperdude