This Time Machine icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,400 icons, all in the same style and quality.
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.