This is the number 9 inside of a box. There is a line drawn across the top of the box, about a quarter inch from the top. There's two little tabs at the top of the box.
This Calendar 9 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 5,700 icons, all in the same style and quality.