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How about to-do items?

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.
 
I think the app requires one to enter the start and end time.


Which can be the exact same time. Are you just wanting to set up a daily to-do list or some generalized list?

Set it up as an 8:00 am start/end each day and list all the stuff you need in notes very easy
 
Which can be the exact same time. Are you just wanting to set up a daily to-do list or some generalized list?

Set it up as an 8:00 am start/end each day and list all the stuff you need in notes very easy

Or you just mark it as ‘all-day’.
 
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