Till When Can One Make an Intention of a Nafl Fast?

1. For a Nafl fast it is permissible to make the intention till an hour before midday. [Note: Midday here does not mean 12pm. Midday in the Shariah refers to half the total amount of daylight hours which is calculated from Subh Saadiq till sunset.]
2. So, for example, if one did not have any intention of fasting till 10am and one did not eat or drink anything till that time, then one felt like fasting and fasted, the fast is valid.

(Behshti Zewar)