How do I set a reminder for every 20 minutes?

A Windows-to-Mac migrant wants to set a reminder on her MacBook Pro/Mavericks to appear every 20 minutes. In Windows, I used Task Scheduler to launch a text file "Stand up and stretch!" every 20 minutes. It would appear, and after stretching, I'd dismiss the reminder by closing the text file and continue working.

The OS X Reminder app allows for a task to repeat daily, but not every 20 minutes, or even hourly. Also, I think those reminders would pop up on my iPhone, where I DON'T need or want them; I only want them to appear on my computer.

Mavericks includes LaunchD, but from what I'm reading online, this is intended for launching System business (daemons, agents...) at startup, and appears to involve using Terminal to issue Unix commands, which is more "geeky" than I am comfortable with.

In the App Store, there is a LaunchD Task Scheduler, which may be a more straightforward solution; I assume it uses a GUI to set up instructions for LaunchD.

Should I try the "LaunchD Task Scheduler" app? Or is there another, simpler way to launch a text file or other type of reminder every 20 minutes?

Thanks.