The issue with months exists in the Java-Script TMINUS.JS that generates the countdown string.
As T-Minus as my first module for DNN - TMinus.js is the first bit of Java-script programming I had / have attempted. So I am not surprised there is an issue. But of interest, you are the second one to mention in 6+ years.
I have placed an un-compacted version of the script in the free TMinus downloads. Please take a look and let me know where I went wrong. Please be kind as this was a first attempt.
Paul.