Sunscreen, as far as I know, is for skin protection from UV radiation by sun, not for whitening so better to listen the professional (esp, the woman since they seems more advanced in this kind like @athayanezant).
@Evans01 true. and I'm more putih langsat than kuning langsat. my ancestors is kinda... Nihon and Chinese but we also have Dutch from grandma.Since SEA basic skin tone is brown and some got "kuning langsat" (how do I said it in English tho?) so I think there's no real white people in Indonesia except for descendants from noni-noni Belanda, or Chindo, or some others. It's just got brighten since skin tone is basic inheritance of our bloodline.
also, SPF 30 PA+++ is good enough for a sunny day but SPF 50 PA++++ is the strong one to protect your skin.
reapply your sunscreen after 4hours and still don't get yourself under a direct sunlight at 12 - 2pm.