This is a Minus 1 Day Filled 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.
It's what looks like a small, desktop calendar, with two rings on the top to change the date. It has "- 1d" meaning to subtract one day, written on the front of the calendar, to be an icon representing the subtraction of a day for other calendar purposes.