Following the disussion here https://forum.plantuml.net/17155/include-date-in-the-milestone-label, are there concrete plans to implement such a feature (in the form of built-in functions, I guess)?
In addition to the scenarios described in the linked thread, I could see such functions useful in, e.g., coloring milestones or tasks depending on whether they occured in the past or will occur in the future.
