This is a Plus 1 Day 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 12,600 icons, all in the same style and quality.
The icon is depicting a calendar that is rectangular with sharp edges and a line dividing the top portion from the bottom. In the center of the object is written plus one 'd' indicating plus one day on the calendar.