“T is one and the same Nature that rolls on her course, and whoever has sufficiently considered the present state of things might certainly conclude as to both the future and the past.”
Book II, Ch. 12. Apology for Raimond Sebond
Essais (1595), Bartlett's Familiar Quotations, 10th ed. (1919)
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Michel De Montaigne 264
(1533-1592) French-Occitan author, humanistic philosopher, … 1533–1592Related quotes

The Notebooks of Leonardo da Vinci (1883), XIX Philosophical Maxims. Morals. Polemics and Speculations.

Source: "Some Perplexities about time: with an attempted solution" (1925), p. 149. as cited in: Jonathan Gorman, "The transmission of our understanding of historical time." Historia Social y de la Educación 1.2 (2012): 129-152.

“The past, the present and the future are really one: they are today.”
Attributed

Thích Nhất Hạnh here quotes and interprets the "Ten Penetrations" of the Avatamsaka Sutra
The Sun My Heart (1996)
Context: The tenth penetration is, "All times penetrate one time. One time penetrates all times — past, present, and future. In one second, you can find the past, present, and future." In the past, you can see the present and the future. In the present, you can find the past and future. In the future, you can find the past and present. They "inter-contain" each other. Space contains time, time contains space. In the teaching of interpenetration, one determines the other, the other determines this one. When we realize our nature of interbeing, we will stop blaming and killing, because we know that we inter-are.

Wars I Have Seen (1945)