The past is gone. But the Present has become the Past before we can know it, i.e. before the complicated phenomenal processes of sense-perception, transmission and conception have been completed. Therefore the Present has gone too.
And the Future? We cannot know it until it has become the Past - for it can never be known in the Present. Then how can it be at all, for we cannot know the Past (which is gone)? Surely we cannot: neither Future, Present nor Past can we ever know.
How, then, do they exist - if existence they have? And if any of them exist, which exists? Or do all of them exist as a unity unextended in time and space, a time and space which only come into apparent existence with them, hypothetically, in order to render them cognisable?
Clearly none of them exists as a thing-in-itself, as objective events in their own right, as phenomena separate from the cognisers of them.
Future - Present - Past appear to be three illusory aspects of a single subjective phenomenon known as 'cognition'.