Minus 1 Dzień and other 86,400 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
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.