Hi Gowtham,
What is the data type of Date field in the OData Service? Ideally it should be Edm.DateTime. For such data types you need to pass date as Date Object. Use as below and it should work.
"RiskValidFrom": "/Date(1449039257586)/"
Here, I have given the date in milliseconds. You can get human readable date by using -
new Date(1449039257586).
BR.