please find a nicer icon for your program matching the style of Xfce panel. Even this one https://github.com/danlucraft/redcar/raw/master/share/icons/calendar.png coming from http://famfamfam.com/lab/icons/ is nicer.
Yes, I can reproduce this in 4.6.1, but it works correctly in the SVN version.
wrong bug updated. I have invested more to the variable icon, which is visible in the tray and some other places. I do not think it is worth investing more time on this static icon.