Answer:
This depends on your employment agreement with your employer. Your invention is your intellectual property so it belongs to you, but if your employment agreement contains a clause where you agree to assign any intellectual property created during the course of your employment, then yes, your employer will own the invention/intellectual property you create during the course of employment. A clause like this is typical in employment agreements (particularly in industries where intellectual property is often generated).
Unless otherwise agreed, the employer would not own any intellectual property you create prior to the terms of your employment contract.
