I'll hafta look into the possibility of parsing the XML data (I'm assuming it's XML format) from the Blizzard Calendar.
Actually, it should be even easier than that.
They reset on a known schedule. Just create a repeating "NOTE" item for calendars.
From
http://www.worldofwarcraft.com/calendar/Blackwing Lair: Every 7 Days during weekly maintenance
Molten Core: Every 7 Days during weekly maintenance
Temple of Ahn'Qiraj (40-man): Every 7 Days during weekly maintenance
Naxxramas: Every 7 Days during weekly maintenance
Onyxia: Every 5 Days at 3:00AM
Zul'Gurub: Every 3 Days at 3:00AM
Ruins of Ahn'Qiraj (20-man): Every 3 Days at 3:00AM
Ideally though if you could grab Data from Blizzard and even put those nifty graphics or icons on the calendar that would be best, because before too long even the 5 and 10 man instances in BC will multi-day instance timers. Woo!