I noticed another peculiarity of the calendar is that the calendar will not automatically account for planned time shifts relating to moving to daylight savings time and vice versa. This is an important consideration if you are in standard time but you are creating an event (using the N900 calendar) that falls in daylight time, and vice versa.

So, again it looks like you have to be cognizant of the "here and now" nature of the calendar: not only do you have to make sure you convert the time to your current time zone when you enter events that take place in other time zones, you also have to convert if you are entering an event further into the future if it falls in a different daylight/standard period.

Alternatively, for daylight/standard, I think it is safer to enter those future events in outlook or another application that does account for DST, and then sync it onto the n900. Those future events, will be an hour off in the N900 calendar, but I presume that they will eventually display the correct time once the N900 shifts to the new time period.