By yesterday I partook in a meeting where an OPEN SOURCE calendar product had been presented. Now, this morning I gave Xlinesoft´s CALENDAR live demo a deeper look in regard to one particular question raised by one of the group, here to find a feature which is making it very transparent to calendar users by either colorization or symbol once an already entered event date had been changed, and found, it could be certainly easily implemented into one of the next calendar versions from your end. Or in other words, there are 4 different levels of calendar planning, annual from LEVEL A users, enter event data where LEVEL B, C and D users have to orientate in their regional planning, same refers in the next hierarchy, say LEVEL B entries are mandatory to LEVEL C and D users. And it often happens that LEVEL A entries change and then the event planning game is re-starting at all levels, hence my feature request to get CHANGED pre-planned events into a more eye-catching shape for easier orientation purposes. Certainly a feature which is of general interest to calendar users with a similar hierarchic data planning structure than mentioned above.
GreeTz - Hajo
P.S. Another way out could be E-MAIL ON EVENT CHANGE with some masterpiece of code as described under PHPR forum section, topic: CONDITIONAL E-MAIL (here to 4 LEVEL user groups).