I use two formulas that return Min and Max date of children to create a “total span”.
The Max date gets an entire day added to it.
I have not done further testing on when this happens.
Replication:
Feature:
ETA: Date with Start and End.
Milestone:
Start Date: Formula with Features.Min(ETA.Start)
End Date: Formula with Features.Max(ETA.End)
Result visualized using timeline: