Non-standard dates on the wiki will screw up the semantic tagging. There are lots of features to the semantic tags that make some pretty cool timelines and tricks available, but they only work with rl style dates.
This kind of stuff is neat for rp, but pretty impractical for regular usage. That's partly why they're is no official calendar for the game. It's mostly good good histories but not daily usage.
Using a template you can store the real life dates for the semantic features while displaying the user defined calender on wiki pages if desired. They are a straight conversion for display only, you still enter the standard dates into the wiki template when configuring it.
