Thanks for the suggestion.
With last beta http://beta.plantuml.net/plantuml.jar you can now have :
@startgantt
Project starts the 20th of september 2018
sunday are close
2018/09/21 to 2018/09/23 are colored in red
2018/09/21 to 2018/09/30 are named [Vacation in the Bahamas]
today is 30 days after start and is colored in #AAF
[Foo] happens 40 days after start
[Dummy] lasts 10 days and starts 10 days after start
@endgantt
or :
@startgantt
Project starts the 20th of september 2018
sunday are close
2018/09/21 to 2018/09/23 are colored in red
2018/09/21 to 2018/09/30 are named [Vacation in the Bahamas]
2018/10/01 and 2 days are colored in lightBlue
[Prototype design] as [TASK1] lasts 38 days
[TASK1] is colored in Lavender/LightBlue
[Prototype completed] happens at [TASK1]'s end
[Release of new album] happens at the 29th of September 2018
today is 2018/10/10 and is colored in #AAF
@endgantt
Note that by default "today" is set to the actual date, so you can also simply use:
today is colored in red
Tell us if it's what you were expecting and if you have other ideas!