App-venture Time Icon Set
Actual license: “Made these for fun today, feel free to download them to use however you like. They make great avatars! Just don’t edit them please, and credit me if someone thinks they’re mathematical.”
Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.