skills : when changing the skill level of a ressource a begin date is mandatory

Hello
I'm running projeqtor 10.0.6 and  I try to wee what I can do with the new the 'skills' (competence) feature

When I go into a ressource and I assign a skill to a ressource, I juste have to enter a skill and a level. Let's say I entered skill = java and level = beginner . it works fine

Then I want to change the level : for instance to expert : I cannot save, the system wants me to provide a beginning date !

It seems to be only when I want to upgrade the skill (go from beginner to expert ) If I want to downgrade the skill (go from expert to beginner) there is no problem

Also If I set the skill to expert and provide a beginning date, I can save , then if I come back and modify the date to erase the beginning date the system allows me to save

Regards

Yes , several :
- why is the date mandatory when upgrading a skill and not when entering a skill for the first time ?
- why when I upgrade a skill and enter a date because it is mandatory and save the data I am I allowed to come back and modify the data to supress the date and save ?

regards

OK I get it thank you