This is a Time Machine icon. We've made it in iOS style, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 12,300 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.