I have improved/revised this in the latest beta/current release.
http://www.flashfxp.com/download-beta/
"until" is now a checkbox to limit the duration and when "until" is unchecked the task runs every X minutes indefinitely.
With the latest build above you can set this as a one time event and it will repeat without problems.
(this wont work with the build you're currently using)
If you don't upgrade then you would need to set the task to run daily starting at 12:01am and running for 24 hours to work correctly.
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