I have the same issue and have just finished a live chat with HTC. The unofficial solution that the agent gave me was to create a google calendar and sync outlook to the google calendar and then sync google with HTC phone....he assures me this will work but was anxious to say it was not an official work around. I pointed out that using another calendar was NOT a solution and asked when HTC would be fixing this. He escalated the problem to technical department who told him it was a known issue with the latest HTC sync manager that "they are working on" but could give no ETA for a fix. Apparently I should keep checking for HTC sync manager updates!
Utter madness.....how difficult can it be? As you say we know when leap years are going to be and I believe the problem only occurs in appointments from Dec to Feb running into a leap year. Definitely considering a change of phone as I use outlook and the phone for my business and can't afford to turn up to appointments a day late.
