[SOLVED] Screen customization : save date on reaching status

Hello,

For some detailed tracking, I would like to add a customization to automatically save the date when a custom status is reached. Same as what is done for 'done date', 'handled date', etc.

I've succesfully gotten this far :
This works if the field is read-only, but I would like to save the value while allowing a user to update it manually if needed.
Is that possible ? 

Thanks