How do you deal with uncertainty when developing for something new that you haven't done before and set a timeline for it?

Despite being a Web developer for a few years, there are always features, like front-end animations (think those cool looking ones from codepen), and back-end scalability, and fail safe capabilities that I don't have the confidence of tackling. Especially when it's something I haven't done before. Giving a timeline for that is next to impossible other than making a good guess. How do you guys deal with this?