Idea of Time Machine Icon
The past icon is a represented with a clock. Instead of a complete circle like most clocks are, the clock is shown with an arrow that makes a circle. However, the arrow is going in a counter clockwise motion to show that it is going backwards or in the past.
This page shows the different variations of the Time Machine from an Icons8 icon pack. Feel free to resize and recolor the icon, and then download the icon in PNG format for free.