Having to give time estimates for custom projects involving functionality unlike anything you've done before - FUCK OFF. How is that possible?!

That's the art, though. It's about breaking it down into small enough chunks that you can reasonably guess (because it will still always only be a guess) how long you'll need. Estimating is a skill like any other and it's no fun if you only get it estimate things you know!!

@CrankyOldDev amen. And resisting to answer the question doesn't make you learn the skill. Practice makes perfect.