How can users generate reminders?

So, I'm about to make a landing page where the users should be able to remind themselves to do a task in the future. They can also do a task right now, but they can choose to do it later. Do you know of a tool or a service that does enable this kind of function? The reminder might be an appointment in their calendar, but the reminder timestamp must be relative to when the user generates a reminder. Ex: 3 months after the user generated the reminder. I would appreciate any tips on this. Thanks!