There are some types of slow cookers that do have timers.
However not all slow cookers have timers and a basic slow cooker may just have a knob to turn the slow cooker to low, keep warm, high or medium heat.
The cheaper slow cookers are basic and do not have timers while more expensive slow cookers may contain timers and may have digital clocks and buttons etc.
However even with a slow cooker you can plug it in, turn it on add the food and leave it overnight or while you're gone to work as long as you put enough water in the slow cooker.
Some slow cookers have timers for low, high, and warm settings, and there are even WiFi enabled slow cookers that let you manage the time with your mobile phone.
Those models with programmable settings are the best for allowing you to be flexible in when you start the cooking cycle and when the meal is ready to serve.
Slow cookers are designed to be left to cook for long periods of time, so the truth is that it's entirely safe to leave your slow cooker on overnight, if you're out the house or if you're at work all day, as long as you follow all the directions and the manufacturer's instructions.