Like KALLT mentioned, tasks (to-do) are now in the Reminders app and do not show in Calendars. There are paid, third party apps like Fantastical and Busycal that integrate tasks into the calendar. I use Fantastical, but it is not cheap.
Usually I just use Calendars and Timepage. I worry that using too many calendar apps may mess up the real schedule due to potential compatibility issue. Do I have to worry?
Usually I just use Calendars and Timepage. I worry that using too many calendar apps may mess up the real schedule due to potential compatibility issue. Do I have to worry?
TimePage appears to use its own database, and does not touch OS X Calendar or Reminders data at all, so I don't see how it could hurt either.
Other apps like Fantastical I mentioned actually use the OS X Calendar and Reminders and iCloud database, so yes in theory could corrupt that data I suppose. That said, I have not had any issues with Fantastical. I actually like it because it does use iCloud data. So I can enter a task in Fantastical on my MacBook and shows up in the Reminders app on my wife's iPad for example.