For a single address with a single output, the score is the product of the number of bitcoins and the number of days since the output was created.  Where there are multiple outputs, these scores are summed.

One's score increases with the number of bitcoins held as well as with the number of confirmations on these held bitcoins.

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Example (at the time of this post):

Address: 1PHudb4xcvhwrVhPnWatrs7mnyzXyj2GEU
Outputs: 1
Value: 0.71020249 bitcoin
Age: 15 days, 13 hours, 40 minutes = 15.569444... days
Score: 0.71020249 bitcoin * 15.569444... days = 11.05745821 bitcoin days (8.d.p).