This is a Date Span 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 12,800 icons, all in the same style and quality.
There are two rectangles. In the bottom rectangle, there is a double-sided arrow. The arrow is in the center of the bottom rectangle. The top rectangle is connected to the bottom rectangle. There are two square on top of the top rectangle. They are to the center left and center right of the top rectangle.