"František is not at home today."
Translation:František dnes není doma.
Because it sounds strange. Even when strongly stressing "dnes" it is still strange. But I don't which specific rule would be against it. The word order and especially the position of adverbs is different in Czech and in English.
Please see above. Read existing comments before asking. It is theoretically possible but very strange.
It would be more likely for a positive sentence: František je doma dnes(, a ne zítra). Still it certainly isn't the normal neutral word order.