Also, I think adding relative start dates is essential. Something like "[Task 1] start in 3 days and lasts 6 days". Otherwise, how can one specify that an activity starts in the future when it is NOT dependent upon some other task (which seems to be the only way to have a task start on Day 1 of the Gantt chart).
To me, this is more important that having absolute start dates for activities. Absolute start dates can also be helpful, but if a project falls a bit behind or its start is delayed, one just has to tell PlantUML that the project start date has changed and then the start dates of various activities change automatically. This would not be the case for absolute start dates, which would each have to be updated manually.
Same applies for milestones, that is, support for either absolute or relative start dates would be ideal. But with milestones, the absolute start dates might often be more important than relative ones.