How to bring Date format in -  \"/Date()/\"

As per one process, i need to overwrite (update) endDate field with a null value once a employee revokes his resignation in the system. And while passing the payload to update endDate value, three fields are mandatory - userId, seq and startDate. There is no issue userId and seq values, but when passed startDate parameter value in this format "2018-12-28T00:00:00", it gave error - "Illegal DateTime value in JSON format: /Date({property.start})/. Please follow the standard format: \"/Date(<ticks>)/\", <ticks> = number of milliseconds since midnight Jan 1, 1970. Example: \"/Date(1495746637000)/\".

So can anyone help in writing correct date format for startDate as mentioned in the error while passing the payload itself.