Windows 10's Timeline is a way of making it faster and easier to go back in time--not just on your PC, but (eventually) on any device connected to it via the Microsoft Graph.
One of the most annoying things about updates in Windows 10 is that you can't rely on the applications you were using opening back up when Windows restarts. So when an update restarts your PC overnight, getting back to where you were and what you were working on takes time. The Windows Timeline, which arrived in the 1803 release of Windows 10, is a way of making it faster and easier to go back in time -- not just on your PC but (eventually) on any device connected to it via the Microsoft Graph.
Office 365 already does something similar across different devices. If you create a Word document on one PC, you'll see it in the list of recent files when you open Word on a PC, Mac or iPad, or when you visit the OneDrive website. The idea of the Windows Timeline is to extend that to multiple applications, first in Windows and then across other devices.
SEE: Software usage policy (Tech Pro Research)
Back in time
When you use the Windows task switcher (by clicking the icon on the Windows taskbar, pressing Windows-Tab or swiping up on the trackpad with three fingers), as well as large thumbnails for all the currently open windows you get a Timeline scrollbar on the right. Move down to see applications and individual web pages (including PDFs) that you've had open in the last couple of days. If you had a lot of activity on one day, you'll see what Windows considers to be the top activities (which seems to be based on how long you were active in the app or how long you spent on the website). Click the link that tells you how many activities Windows recorded to see the longer list, grouped by hour and sorted in reverse order.
You can also use the search box on the top right to search for an application like Windows Paint and see all the files you've had open in it, or for keywords in the title of documents or on web pages you've visited. This doesn't search inside the content of documents, but it's good for finding files or searching your browser history -- something Edge still doesn't let you do.
If you want your Timeline to go back further, scroll to the bottom of the screen and click the 'Turn on' button under 'See more days in Timeline'; this will also sync activity on your PC through your Microsoft and Office 365 accounts. If you want to turn the cloud syncing back off, you can do that from Settings/Privacy/Activity History (for one or all of your accounts).
You'll be able to scroll back to see applications, files and websites from the past 30 days on your current device almost immediately; it can take up to 24 hours to see details from other devices there as well (although we started seeing results in as little as ten minutes in some cases).
Even if you don't see an activity from another PC listed in your Timeline, it will show up when you search by name. If you use your PC for a lot of things at once rather than carefully opening a set of documents and web pages to work on together, searching is going to be the most efficient way of using the Timeline to find things anyway.
Once you find the document or site you want, you can click on it to open it. Right-click on an activity to remove it from your Timeline, or to clear all activity for that day.
If you don't find anything useful, or you triggered the Timeline view by accident, you can close it by pressing Windows-Tab again, or the Esc key (if you've scrolled down the timeline or opened more details of a specific day, each time you press Esc you'll go back up a level). Or you can click anywhere on the background rather than one of the activities to close the Timeline view immediately.
Cortana offers a smaller snapshot of your Timeline, if you tap or click the Cortana search box in the taskbar but don't start typing anything. Click 'See all activities' to jump into your Timeline, skipping past current open windows and straight to the Earlier Today section. (In the next version of Windows, 1809, that link is renamed to Open Timeline.)
Not just PCs, but not many apps
If you're using the Edge app for Android or iOS, web pages you open on your phone or iPad will show up in the Windows Timeline on your PC, so you can open them there. They're not labelled as pages you looked at on your phone, so you can't use that as a clue to find something you want to get back to. The mobile Edge app also lets you use the 'Continue on PC' option to open a web page in the default browser on your PC (if you have several, you can choose which one to use). That's handy if you need to fill out a form and want to do it with a real keyboard.
The Microsoft Launcher app on Android shows you a list of local apps that have been updated recently (which you can't see on your PC), as well as Office documents you've opened on other devices, so you can get at them quickly on your phone. That will come to iOS "later this year" according to Microsoft. Launcher also adds Continue on PC to the list of share targets in Chrome, so you can push web pages from there to your PC.
So far, Edge is the only desktop browser to have native support for the underlying Remote Systems APIs (originally called Project Rome) that make the Timeline work. However, you can add Timeline support to other browsers by installing the Timeline Support extension for Chrome, Vivaldi and Opera, and Firefox -- including Firefox Mobile. This adds pages you browse to your Timeline, and you can change the length of time you have to stay on a page to have it show up on the timeline from the default 8 seconds if you find that too short. Install this extension in Chrome on a Mac or Chromebook, or in Firefox Mobile on an Android phone, and you'll see pages you browse there in the Timeline on your Windows devices. You can also use it to push a web page you're looking at on one device to open on another device where you use the same account.
SEE: Windows 10 April 2018 Update: An insider's guide (free PDF) (TechRepublic)
So far, only a limited number of apps show up in the Timeline: Office applications like Word, Excel, PowerPoint and Access appear, but not Outlook or OneNote (not even the Windows Store version of OneNote). You see files you've opened in Paint and Notepad (but not Paint 3D), and any images you've opened in the Photos app if you open them from Explorer -- but not if you browse to them inside the Photos app. You can see locations you've searched for in the Maps app and individual stories you've read in the Microsoft News app.
Xbox applications can put items in your Timeline -- Microsoft presentations suggest Minecraft will be the first example. And Office documents you create and edit on a Mac, Android or iOS device where you use the same Microsoft or Office 365 account will appear in your PC Timeline too.
Other apps are starting to add support so that they show up in your Windows Timeline: so far that includes the myTube YouTube app (which is in the Windows Store). Developers can choose what counts as an 'activity', which is why the official Twitter app for Windows adds any live events and Twitter Moments that you look at to the Windows Timeline (although not individual tweets or videos, as that would presumably clutter up your Timeline).
You can also see a much more comprehensive list of your Windows activities -- far more than show up in the Timeline -- by going to the activity history in the privacy dashboard of your Microsoft account. This shows what you've asked Cortana, apps and services (including push notifications, printing documents, using the device camera, opening the Photos app, the Store and pretty much every other app on your PC) and locations where you've used devices when you've used an app with location permission. If you're still using Windows Phone, which doesn't put websites into the Windows timeline, this is a handy way to bring websites you've browsed onto the phone to your PC -- or to get at activity on a Windows 10 PC from an older version of Windows that doesn't have the Timeline. It doesn't have the modern interface, but this online list might actually be more useful.
- Windows 10 updates are about to get smaller and less painful (TechRepublic)
- Windows 10 April 2018 Update: How to delete activity history from your Timeline (TechRepublic)
- Windows 10 April 2018 Update: How to turn off suggestions in Cortana and Timeline (TechRepublic)
- Windows 10 after three years: A greatly improved report card (ZDNet)
- Securing Windows policy (Tech Pro Research)
- Windows 10 to get new 'InPrivate Desktop' security feature (ZDNet)
- This is not your father's Microsoft (CNET)