Date fields should use the date in the Value, not additionally use StartTime and End Time Primitives

[jrippingale]

TaskCompletedDate currently uses this Sample Value and Usage Tips:
Usage Tips: Use EndTime for the actual completed date. StartTime should be the task start date
Sample: { "EndTime": "2020-11-30", "StartTime": "2020-05-30" }

Because the concept name represents a single value, this should have one value and be used like this:
Sample: { "Value": "2020-11-30"}

Similar issue with TaskDueDate