Derek Moran
Bitcoin days destroyed for any given transaction is calculated by taking the number of Bitcoins in a transaction and multiplying it by the number of days it has been since those coins were last spent.
Example
If someone has 100BTC that they received a week ago and they spend it then 700 bitcoin days have been destroyed. If they take those 100BTC and send them to several addresses and then spend them then although the total transaction volume could be arbitrarily large the number of bitcoindays destroyed is still 700.
https://en.bitcoin.it/wiki/Bitcoin_Days_Destroyed