Timeline was supposed to make it into the Windows 10 Fall Creators Update but Microsoft didn't get it ready in time. At yesterday's Windows Developer Day, the software giant revealed Timeline is now scheduled for Windows 10 Redstone 4. This means it's expected to be released around March or April 2018.
A beta version of Timeline will be made available to Windows Insiders soon. MS Power User offers some screenshots and extra info:
Windows Timeline doesn’t look a lot different from some of the concepts Microsoft showed off earlier this year. As it can be seen from the screenshots, Timeline will be accessible from Windows 10’s Task View button. You will also be able to use the Virtual Desktops feature in Windows 10 from there. There’s a search button which you can use to search all of your activities and apps from the past.