What you are seeing is quite weird. In my 6.1.1 board I went and changed my birthday from April to January. Immediately in my calendar board the entry from April disappeared and appeared in January.
What the calendar modification on the profile birtdays does is it queries the users table for birthdays on a particular month and while the script runs to display the data in the calendar it also displays the birthday. The w3t_events table is never used for birthdays.
Very weird that your users see the events.... Did you or your users manually entered events on the calendar board? To check this, see the color of the three little arrows in front of the person's name on the particular date. If they are blue, it was generated by this modification if Red then it was manually entered in the w3t_events table as a new event.
Any chance of having a sample to have a look at the behavior?
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