While acknowledgements typically have a lighter tone than the rest of the dissertation, it doesn't make writing them something you finish within 20 minutes.
So when you are planning to write your chapters, you need to understand that compiling everything into the entire dissertation might take some additional time. You'll need to add not only your acknowledgements, but also your CV and make sure everything is in the same layout.
My colleagues told me to write my acknowledgements early on, and I followed that advice. I wrote my acknowledgements in November, as I was preparing my first draft. And I'm very glad I did so before getting sucked into the frustrations and anger/sadness that can arise at later stages in the writing/editing process.
I've restated a few sentences, and I've added some people that I forgot to name, but the core of the text hasn't been altered since November.
Therefore, if you are writing your chapters, don't forget to write your acknowledgements!