You should implement an option list in the calendar options page that allows you to change referenced time used to receive calendar notifications between server time, local time, and perhaps a custom time slider with fixed time intervals(...,-2,-1,0,+1,+2...).

- The main purpose for such an implementation is to enable players that are in different timezones and to receive correct notifications relative to their local time.
+ The main purpose for such an implementation is to enable players that are in different timezones to receive correct notifications relative to their local time.
  For example,  I play WoW on an English server, with a Greek guild that posts calendar events relative to Greece local time which has a +1 hr difference with server time. Altoholic, as it is setup by default gives me notifications relative to server time and thus I allways receive them 1hr after the actual event starts.