All except this one!
Long story short, after debugging the heck out of the calendar control, I noticed that the specific date was a daylight saving date, but only for my home country of Romania. Several tests showed that if the date object was instantiated with the year, month and day only, it will set the date to the previous day, hours 23:00. The fix was to set up the date by specifying the hours 12, instead of the default 0. Since then the bug silently disappeared, maybe from a Windows update of some sort. However, I still remember it fondly.
Do you have a bug whose story you love to tell? Let me know!