How to map static date field in Async SOQL query job?

Question

I'm trying to use Aggregate Async SOQL to Query Big Objects to a custom Object like below (https://developer.salesforce.com/docs/atlas.en-us.chatterapi.meta/chatterapi/connect_responses_async_query_output.htm) :

{    
   "query": "SELECT firstField__c, secondField__c FROM SourceObject__c",     

   "targetObject": "TargetObject__c", 

   "targetFieldMap": {"firstField__c":"firstFieldTarget__c", 
                      "secondField__c":"secondFieldTarget__c"
                     },
   "targetValueMap": {"$JOB_ID":"BackgroundOperationLookup__c",
                      "Copy fields from source to target":"BackgroundOperationDescription__c",
                      "2018-01-01T00:00:00Z" : "Field_Date__c"
                     } 
}

I want to make all the fields I query from the big object to have the Field_Date__c = 2018-01-01 hence why I added in the code in the POST request in workbench

"2018-01-01T00:00:00Z" : "Field_Date__c"

However I always get the error :

INVALID_INPUT
message: The target value field '2018-01-01T00:00:00Z' isn't a valid query substitution parameter.

I tried using all these other "acceptable" format below but still got the same error.

'Date' field acceptable formats

• YYYY-MM-DD
• YYYY-MM-DD hh:mm:ss
• YYYY-MM-DD hh:mm:ss
• YYYY-MM-DDThh:mm:ssZ
• YYYY-MM-DDThh:mm:ss.sssZ

'Date Time' field acceptable formats

• YYYY-MM-DD hh:mm:ss
• YYYY-MM-DDThh:mm:ssZ
• YYYY-MM-DDThh:mm:ss.sssZ

What do I DO?