It's now available on Windows 10 and coming to Android and iOS "in the coming weeks." To use it, you'll need enable an Outlook or Office 365 email account to give Cortana consent. The feature is proactive and will automatically pop up reminders on your PC, assuming you've set a time and date. If not, it'll collect them in the Cortana Home view, and let you create your own timed notification that will appear on any devices with the app.

Cortana already has location- and contact-triggered notifications, so as shown in the video below, if could remind you to get milk if you happen to be near the market, for example. Google has done something similar for quite awhile now with its own assistant -- it'll scan your emails for plane reservations, for instance, and post a notice in the Google app telling you when you need to leave. However, Microsoft's application seems a bit more proactive, and it'll eventually work with other email services, including Gmail, soon.