If you open the datepicker and if you change the month, then the current day is selected in each month.
Steps to reproduce:
1) Open: http://aryweb.nl/projects/mootools-datepicker/Test/
2) Open the picker
3) Click on the date to go to the months selection
4) Click a random month
5) Navigate through the months with the arrows
The current day is selected in every month