The time clocks is located on the boiler to control the central heating (normally on Saunier Duval combination boilers). The time clocks brings the boiler on at pre-set times throughout the day so heating isn't on 24 hours a day constantly. Most people set their Saunier Duval boiler to come on in the morning just before they get up, they then set the timer to turn the boiler off just before they leave for work.
This is then repeated for when they arrive back from work the boiler comes on again and goes off just before they go to bed. The time clocks is also referred to as the programmer, timer, time switch or just the boiler Clocks. They come in mechanical and digital versions depending on the customer's requirements and they are now available as an App on smart phone such as Nest & Hive on the latest Saunier Duval boiler models.