Diverse life forms have evolved internal clocks enabling them to monitor time and thereby anticipate the daily environmental changes caused by the earth’s rotation. The plant circadian clock regulates expression of about one-third of the Arabidopsis genome, yet the physiological relevance of this regulation is not fully understood. Here we show that the circadian clock, acting with hormone signals, provides selective advantage to plants through anticipation of and enhanced defense against herbivory and pathogen infection. I found that plants entrained in-phase with the herbivore Trichoplusia ni (T. ni) are more resistant than plants entrained out-of-phase. Similarly, when plants are inoculated with Botrytis cinerea (B. cinerea), the time-of-day at which the inoculation initially occurred significantly affects the rate of infection. Both the circadian clock and jasmonates are required for resistance to herbivory and time-of-day resistance to pathogen infection. Jasmonate and salicylate accumulation is circadian regulated, and accumulation patterns are in opposite phase to each other. I also demonstrate that the circadian clock of post-harvest cabbage (Brassica oleracea) is entrainable by light-dark cycles and results in enhanced herbivore resistance. In addition, entrainment of Arabidopsis plants and post-harvest cabbage causes cyclical accumulation of metabolites that function in plant defense. Finally, I show that the phenomenon of post-harvest entrainment and enhanced herbivore resistance is widespread among diverse crops. The evolution of the plant circadian clock not only provides a strong physiological advantage in pest defense but also can enhance the nutritional value of plant-derived food.