They can indeed go longer. I've had some which were only intermittently brooded by a deficient mother hen last for a month before they hatched. They can also hatch in less time. But sometimes, mainly dependent on the health or lack thereof of the parents, particularly the mother, they can simply fail to start hatching. In which case there's nothing much you can do.
If she's not the sort of hen to tuck eggs under herself as soon as she sees they're not under her, then it's entirely possible they've been left in the cold overnight beside her and it's delayed development and therefore hatching.
I would leave them at least until the 30th and by then you should know for sure. But do whatever you think is best.