Within NaturalHR employees can edit time off bookings up until the time off start date, when a time off booking is future dated it will show as 'Booked':

However once the start date is reached the state will change from 'Booked' to 'Taken':

Once a time off booking has changed to 'Taken' it is no longer editable and can only be viewed by employees. if an employee does need a 'same day' time off booking to be edited HR and admin level users are still able to do so.