Where to calculate totals?

When using custom entities (aka business entities, domain objects, business objects, entity objects) how should you get totals to display on a page?  Take the hypothetical simple example of system that tracks users and the number of miles they have run on a given date.
pseudoclass RunEvent {
    public Date; 
    public UserName;
    public NumberOfMilesRun; 
}
 
And you display a list of these in a gridview like:
6/1/08, Jen, 7 miles

6/1/08, Mark, 5 miles

6/2/08, Roger, 6 miles
etc.
If you want to display the total number of miles run, should you handle this in the database, in business classes, or in the UI layer? 

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