Yes you can put tin foil or aluminum foil in a slow cooker.
You can wrap meat and vegetables in foil, and then place those individually wrapped food items into your crock pot for cooking.
For example, you could wrap potatoes in aluminum foil and then place the wrapped potatoes in a crock pot to cook.
You can leave food cooking in a slow cooker for a few days or so while you eat it.
As long as you keep adding water to the slow cooker it will be okay to leave the food slow cooking or keeping warm for a few days.
Sometimes I will make a large pot of beans with some hamburger meat or bacon etc and leave it in the slow cooker and slow heating for a few days or even 3 days while I eat the beans.
It's never been an issue doing that.
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.