I need to add tracking to some of this posts. Sorry for not answering sooner. I am not sure why this is not working for you. I had previously tried this script to do exactly what you needed to do and it worked. Let me give it a try and figure out why is not working.
Honestly, this script is using a TaskPaper api and its dictionary and @complexpoint writes very concise code that is above my limited knowledge. Maybe he can give it a quick look and see why is not working. I will still attempt to figure out why is not working
In the mean time see if you can use dates differently. I wrote a primer sometime ago about using dates in TaskPaper. Here is the link