Persuade yourself of what I write: Some moments are torn away from us, some are stolen, some vanish away. But the worst loss of time is due to our own negligence. -Epistle 1
The goal of the ancient stoics was to live according to nature. Their understanding of what "living according to nature" means, however, was vastly different than what our modern ears might imagine. We might think it means to shun artificial social conventions; to live in the forest like animals; to reject the city life and make friends with birds and squirrels instead of people; to accept and even embrace our emotions and impulses, even our negative emotions, as natural feelings.
Our modern notions of living in accordance with nature could not be further removed from the ancient stoic view of nature. Our social conventions and the personal roles they entail are not man-made, but part of the fabric of the cosmos. For a man to fulfill his role as a man, a woman as a woman, a son as a son, a citizen as a citizen, a neighbour as a neighbour, etc - these are not artificial constructs thrust upon us by an equally artificial society, but natural relationships and duties prescribed by Nature herself. Nature made us social animals, like bees in a hive, each with his or her own part to play (not chosen roles, but ordained roles), living in communities like men, not in the wilderness like beasts. But the most important distinction between modern and stoic concepts of what constitutes "living in accordance with nature" are their opinions concerning the negative emotions, or the Passions. Anger, Fear, Intemperance, self-indulgent Depression and all the other horrible Vices are not natural emotions. They are unnatural feelings brought about by our own poor reasoning and ignorance of the true Good (Virtue) and the true Evil (Vice).
"The worst loss of time is due to our own negligence." And we are negligent when we fail to live according to Nature. Yet nothing can prevent us from doing so, except our own selves. Our time is always our own if we choose to make it so.