Hi,
For the best results, you should apply sunscreens in the morning about half an hour before leaving the home.
Do remember to reapply every four hours (more frequently if you are sweaty or indulging in outdoor sports such as tennis and golf) to maintain the sun protection efficacy of the applied sunscreen.
In addition to using sunscreens, it is best that you also shield yourself from direct sun exposure e.g. carrying an umbrella, or wearing a sun hat to maximize your sun protection.
Hope this helps!