Although you’ve been able to set your notifications to sort by date since the Notification Center first debuted back in iOS 5, all this option actually did was sort your apps by the date the last notification was received, grouping each notification under app-specific headings. Unfortunately, this didn’t provide an entirely chronological view of your notifications, since apps that had received several notifications over the course of a day would still have those older notifications grouped with it, above whichever app had displayed the second-most recent notification, and so forth.
With iOS 9, Apple finally added more flexibility to see your notifications in an entirely chronological view, regardless of which apps they come from. Although Apple had oddly debuted this new feature as a switch you need to turn OFF, rather than ON, it can be found under a new “Group by App” option in the Notifications Settings. When this option is enabled, your Notification Center will behave in much the same way as it has since iOS 5, but toggling this option OFF will instead present all of your Notifications in a single list, grouped by day rather than by app.
As an added bonus, this also allows you to clear an entire day’s notifications in one fell swoop, since the normal “X” buttons used to clear groups of notifications will apply to each day grouping, rather than groups of apps. Note that for obvious reasons, the Group By App option will only work if your sort order is set to “Recent,” so it’s hidden entirely if you switch sorting back to “Manual.”