Yes! Vitamin C is an antioxidant and it neutralizes radicals. Ultraviolet light emitted by the sun induces free radical formation in the skin and that leads to photo aging and even cancer. Vitamin C is a great antioxidant and it protects the skin against the harmful effects of UV radiation. So it’s not only safe to use Vitamin C during the day, it in fact is helpful to do that. However, please note that Vitamin C is not a replacement for your sunscreen and you should always layer your Vitamin C serum with a good sunscreen when using Vitamin C during the day.