What is the purpose of 'Start Date & End Date'?
The date's is not to disabled your page, however this help to remove the event from the cause page (Nonprofit main page). Yes, whoever has the link they still able to donate or share the page. But if you want the donate button to be to be removed from the event page you will need to 'Disabled' the page from your Admin account.
You can make this changes from the Charity account. Here is the steps;
1-Click on 'Fundraising Event and choose the event
2 - Change the start date and end date that you want
3 - Click save