Like Apple's Handoff, Timeline lets users pick up work where they left off from different devices. Start a project on your laptop and you can finish it on your phone without any file-transferring hassle. But unlike Handoff, which is limited to Apple devices, Timeline will let users switch between Windows 10, iOS and Android devices.
In a tweet, Belfiore said that Timeline definitely wouldn't be in the Fall Creators Update, but that Microsoft was planning for the feature to be included in insider builds shortly afterwards. He went on to add that it wasn't delayed, it just wasn't ready in time and that timing had never been specified at Build.
The Fall Creators Update is expected to begin rolling out in September. The Windows 10 feature update is expected next March.