You can certainly add the chlorine anytime you wish. That said, if you have any organics in your pool, the Cl will be more effective after the Sun goes down.
If your pool is pristine like mine (yeah, right :lol: ) it really won't matter that much as the Sun will simply take it's toll the next day.
THIS IS NOT GOOD ADVICE...... but there could be an argument made for adding in the daytime in an organic-free pool. That way the residual Cl will not deplete as much during the day....giving you a more uniform ppm level thru the 24 hour period.
Few pools exist like that, however, so I think the idea of adding in the evening makes the most sense. Nevertheless, if it's impractical or even inconvenient, I would not consider it mandatory.
(Thanks for the kit order)