About time, part 3: Time and perception

For something that we commonly consider to be as regular and predictable as clockwork, time sure can feel weird. Sometimes it drags, sometimes it rushes, sometimes it seems to stop altogether. We don't experience this skewed perception with other phenomena - with colours, for instance. The blue of the sky looks like the blue of the sky, no matter what we're doing or how we're feeling. So why is our experience of time so variable? Is it something that happens purely in the mind, or does it have something to do with the flexible nature of time itself?