Here's a Sedley paper on that topic.
QuoteAristippus…advised people not to pain themselves either in memory of what is past or in anticipation of future events (μήτε τοῖς παρελθοῦσιν ἐπικάμνειν μήτε τῶν ἐπιόντων προκάμνειν).…His advice was to keep one’s thought focused on the day, and in fact on that part of the day in which one was carrying out this or that action or thought. For only the present is ours, he said, unlike what is already over and what is still awaited, of which the former has perished, while with the latter it is unclear whether it will be.