If you want to have a custom exposure after the timer goes off, you can use the Timed Release and Self Timer mode. This mode combines the Self Timer with the capability of keeping the shutter open for a certain time. It has two parameters: Timer and Exposure.
Timer: This parameter defines when to trigger the camera.
Exposure: This parameter defines how long to keep the shutter open.
Once you set the parameters and press start button (the orange tick located at lower right corner), a status screen will be opened as below.
Firstly Timer will count down from the timer value and when it goes off, Exposure counter will start showing how long the shutter has been kept open. The shutter will close when the custom exposure value is reached.
You can stop the mode by touching Stop button 2 times. When you touch first time, the lock will be opened and when you touch it second time, mode will stop.