Many thanks for your suggestion!
Using last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar
you can now define something like:
Project starts the 20th of september 2017
[Prototype design] as [TASK1] lasts 13 days
[TASK1] is colored in Lavender/LightBlue
[Prototype completed] happens at [TASK1]'s end
We did not use format like 2017/08/01 because, depending on people usage, day and month can be ambiguous.
About syntax, it's not that easy to find a simple one. Several tries are usualy needed to find better ideas.
So do not hesitate to play with our proposal and to make suggestions: nothing is fixed right now, so you can help here!