Olive leaf can be harvested at different times of the year but will have minimal impact on this product because most oleuropein comes from a standardized extract always having 25% oleuropein. Also note research indicates that as oleuropein breaks down into its constituents, the level of hydroxytyrosol increases. Hydroxytyrosol is an excellent healthful antioxidant constituent. This can also happen in the body where oleuropein is consumed and broken down into hydroxytyrosol. More research is needed to determine the relationship between the two compounds but we believe both are very beneficial for health reasons.