On the opportunity card the follow-up date is available. This field can be used to enter a date you have to perform the next action for the opportunity. In the CRM app you can simple indicate whether you want to sort the opportunities on the end date (closing date) or on the follow-up date.
In the overview of opportunities you can see if the list is sorted on end date (closing date) or on the follow-up date on top of the screen. By pressing it you can indicate how you want to sort the information.