When campaign is triggered without dates defined?
The start date and end date just graphically mark the start and end of a campaign. These activities do not have any functional impact and your campaigns/workflows work normal even if the end date has passed.
The start date and end date are used to define the campaign information in Calendar UI
These activities do not have any functional impact on your Campaign Processes.
Even if your End date is expired, Campaign will continue to run as it is.