It’s one of those features which the PC version of Minecraft has by default but is not likely to be an official part of Pocket Edition for a long time.
In this example we will be using the commands available in the SimpleCommands mod to explain how you can use redstone and the Command Block to switch time between day and night.
Then place down the second Command Block and let’s set it to execute the day command.
Now use the levers to trigger the redstone signal. The redstone signal will then transmit power to the Command Block which will in return execute the command it is setup to execute.