Thank you @PramodMohandas as you mentioned the date format working fine in postman.
In Java code also the same date format needs to be handled as below,
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd=HH:mm:ss");
categoryJson.put(category_attribute_id, sdf.format(new Date()).replace("=", "T"));
So that, the date format will be passed in proper format to Java http post request.
Formatted Date using Java code: 2020-09-02T13:30:00
After this date format, it is working fine with Java code too.