Rest API to feed FinishDate for Project Team Members

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I am currently testing the use of Rest API to push changes for project team members in our system. I'm trying to use a spreadsheet populated with the information to push the change, but cannot get a successful update when trying to pull the finish date from the spreadsheet. Using the json below, I can feed ProjectId and TeamMemberId from spreadsheet as long as I am using one specific date for all, but if I try to use the third line to pull from the same source, I get a bad request. Had anyone done an update like this and