Thief of Time: (Discworld Novel 26) by Terry Pratchett
Time isn't a toy to be played with. It's bigger than all of us. It exists outside the artificial and arbitrary divisions into which humankind has presumptuously shoehorned it. It needs to elastic. To understand truly, you have to see time as the most important resource we have. You mess with it at your peril. You can let it move fast or slowly but what you mustn't do is allow it to stop. Because when time stands still, everything stops with it. Then, there really is no future. And where would we be without one of those?