After more than one year, I finally managed it to release a new version of Task Till Dawn. Even though the GUI still looks the same, a lot has changed under the hood.
Added an option to allow tasks to only run between specified times.
New option to start another task if a task was executed successfully.
New option to start another task if a task could not be executed.
Added an option to skip missed tasks.
New option to automatically start Task Till Dawn after login.
New initial setup assistant.
Improved text to speech notifications on OS X.
Better handling of calendars other than the Gregorian calendar.
A tasks delay is no longer added to its execution timestamp.
Already running tasks can now be stopped.
Reduced memory usage.
Signed application to support Gatekeeper on OS X.
Improved handling of the "Hide in Dock" option.
Added basic support for fullscreen mode on OS X Mountain Lion.
Add or edit multiple tasks at once.
Create new tasks using the menubar/tray-icon.
Add new tasks without opening the tasklist first.
Better compatibility with Exposé / Mission Control on OS X.
Fixed a problem which caused a message to appear while shutting down the system.
Fixed a typo in the comments dialog.
Fixed support for the recent Growl versions on OS X.
Removed Growl support from the Windows version.
Some other minor bugfixes