Not able to declare "start date" in screen "Resource cost"

The start date is not updatable for first cost with a given function.
It mean this cost is valid for any date.

When you enter new cost with the same function, the date is manadatory.
This means, fort instance if you have "cost1" (xwith no date) and "cost2" at date "date2" :
cost1 is valid for any date up to "date2"-1
cost2 is valid for any date since "date2" (up to any date in the future)

Hi,

I don't understand your request.
Do you mean entering Start Date of Affectation ?
What do you mean by "enter it in two times" ?

What is sure is that affectation start date cannot be set from resource function, because there is not link between these information.
Affectation start date will define when resource will start working on project.
Function start date defines when resource gets this function or when cost for this function changes.

Moreover, affectation start date is optional.