This Countdown 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 5,500 icons, all in the same style and quality.
It's an icon of the last hour reduced to a clock with a 1 on it. The 1 is centered in the middle of the clock. The clock also has a counterclockwise arrow running all the way to 11 in order to symbolize there is 1 hour left.