This Minus 1 Day 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.
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.