Change Start & Due Dates using a workflow

We want to make Start & Due dates read-only and then use a workflow webservice method to update those fields from custom attributes. I am having a hard time referencing the Start & Due dates. This is what I have so far:
[
   {"op": "add", "path": "/StartDate", "value": {{rsdate}}}
]

When I test the code I get a deserialized error - it doesn't seem to matter if I use a variable or use a date value.

Can you tell me what the issue is?