Most experts agree that chlorination should be done in the evening or at night. The simple reason is that chlorine will have more time to work and kill the bacteria and viruses in the absence of sun.
Fair enough, the stabilized chlorine can offer some shield against the harsh sun rays, but it does not block them completely. This is why it's ideal to add your chlorine tablets to the pool at night. However, in case you have an indoor pool, you can chlorinate any time of the day.