Add Date only if the field is null and prevent further changes

Don,

I don't think I described the situation well enough.  The completion date is actually pulled from an external system that the users do have the ability to modify.  The T-n dates are calculated fields based on the completion date.  The logic we are striving for is at completion date T - n days, if the Completion at T - n days is null, then we want to write the date of Completion Date - n.  But if the Completion Date at T - n days has a value, we do not want to change the date.

As an example, let's assume the current Completion Date is 4/15/2020.  On 3/16/2020 the Completion Date at T - 30 is 3/16/2020.  If the Completion Date at T - 30 IsNull, then the date of 3/16/2020 will be entered in Completion Date at T - 30.  If it already has a value, then the date must not be changed. 

The intent is to prevent the users from "gaming" the system to make it appear they are hitting a higher percentage of on time completion by changing the Completion Date after the Completion Date at T - n.  Following the example above, suppose on 3/18 the user notices they missed the Completion Date at T - 30 and they change the Completion Date to 4/17/2020.  If the Completion Date at T - n days can be overwritten, the users can manipulate the data to make the on time completion rate appear better than it really was.