How to capture comments when milestone start date changed on Custom Panel view?

Hi 

I want to capture comments when milestone start date changed on Custom Panel view. I had created  Account Milestone view to see all milestones as a flat view on that view i want to Maintain Audit log of Start date in comments column against that milestone - In comments user enters reason why the start date is changing. 

OR Is there any other way to Capture comments & Save it?.

Requirement is like below- 

1) Ability to capture comments when milestone start date is changed.

2) Ability to view for every single milestone – the details of when start date were changed (audit log)

Thanks,

Amit.