Fill in a field with the modified date/time modified date/time of another field

Hi,

I am trying to update the "Contract_Returned_Signed" field with the date/time that the "Opportunity_Status" was modified, but I am getting the error message "Failed to execute function Argument type mismatch - Found 'TEXT' but Expected '[BIGINT]' for the function 'get' Line Number:6".

Please advise on correct script.

1 thisRecord = zoho.crm.getRecords("Contract_Management",recordID);

2 info recordID;

3 mp=map();

4 mp.put("Contract_Returned_Signed",ifnull(thisRecord.get("Modified Time"),"Opportunity_Status"));

5 update = zoho.crm.updateRecord("Contract_Management", recordID, mp);

6 info mp;

7 info update;

Thanks,

Jason