Vaisesika Sūtra 2.2.06

अपरस्मिन्नपरं युगपच्चिरं क्षिप्रमिति काललिङ्गानि ॥६॥

aparasminnaparaṁ yugapacciraṁ kṣipramiti kālaliṅgāni ॥6॥

[अपरस्मिन् (aparasmīn) - in the other; अपरं (aparaṁ) - is before; युगपच्चिरं (yugapacchiraṁ) - simultaneously; क्षिप्रम् (kṣipram) - immediately / the current; इति (iti) - thus, in this way; काललिङ्गानि (kālaliṅgāni) - indicative of time;]

Having simultaneous experience of the other as before, and the current moment as now, is the distinguishing mark of time.