From this comment: https://news.ycombinator.com/item?id=15831385 https://news.ycombinator.com/item?id=15831385 Basically, you can do notifications up to 30 day

From this comment: https://news.ycombinator.com/item?id=15831385 https://news.ycombinator.com/item?id=15831385 Basically, you can do notifications up to 30 days ahead, 2nd December + 30 days = 1st January, something overflows and thinks month(January) = Month(December)+1 instead of correctly looping around.