One of the features that Microsoft had promised for the Fall Creators Update at its announcement earlier this year was Timeline, which lets you look back in time and see what you were doing at a particular time.
Unfortunately, contrary to Microsoft's initial promise, the feature did not make the cut for the Fall Creators Update due to a lack of time but, as Kevin Gallo, the head of the Windows Developer Platform, showed at the company's Developer Day event, the feature is already up and running on internal Microsoft builds. This means it is well underway for its eventual debut in early 2018 with the Redstone 4 update for Windows 10.
The implementation of the feature is similar to the concepts Microsoft showed at Build 2017. You can access Timeline via the Task View button and there also seems to be a search option to look for your usage history of apps, though it's not yet clear how long Timeline's memory will be.
Gallo also promised that Windows Insiders will be able to test it out for themselves very soon. The demo in question was on Build 17005 and, to put this in context, the RTM build for the Fall Creators Update that most Insiders have installed now is numbered 16299.15. After the official launch of the update on October 17, Microsoft will likely make the difference in no time as the Skip Ahead ring of Insiders is already testing Build 17004.