Why can't I modify the harvest period of a coffee harvest?

The harvest start date is used as an identifier of the harvest. It will appear in the title of the coffee - so you are not able to modify it. 

If you want to modify it because a new harvest is starting, please mark the current harvest as 'past' and create a new harvest instead. All the coffee lot data will be filled already, you only need to update the quality and price to start selling your harvest.