var ss = SpreadsheetApp.getActiveSheet();
var dateRange = ss.getRange(2,7).getValue(); // Value in Cell G2 is "7/17/2018"
Logger.log(dateRange); // Returns Mon Jul 16 20:00:00 GMT-04:00 2018
Logger.log(Utilities.formatDate(new Date(dateRange), Session.getScriptTimeZone(), "yyyy-MM-dd"));// returns 2018-07-16

var ss = SpreadsheetApp.getActiveSheet();
var dateRange = ss.getRange(1,2).getValue(); // Value in Cell B1 is "7/17/2018"
Logger.log(dateRange); // Returns Tue Jul 17 00:00:00 GMT-04:00 2018
Logger.log(Utilities.formatDate(new Date(dateRange), Session.getScriptTimeZone(), "yyyy-MM-dd")); // Returns 2018-07-17