question: private long cacheResponseAge ( ) { long apparentReceivedAge = servedDate != null ? Math . max ( 0 , receivedResponseMillis - servedDate . getTime ( ) ) : 0 ; long receivedAge = ageSeconds != - 1 ? Math . max ( apparentReceivedAge , SECONDS . toMillis ( ageSeconds ) ) : apparentReceivedAge ; long responseDuration = receivedResponseMillis - sentRequestMillis ; long residentDuration = nowMillis - receivedResponseMillis ; return receivedAge + responseDuration + residentDuration ; }
target:  Returns the current age of the response, in milliseconds. The calculation is specified by RFC 2616, 13.2.3 Age Calculations.