Bruno, My understanding from the debevec.org site was that they aren't taken at different exposures at different moments of the day and combined as a single HDR image.  The are a highly detailed info-laden panaramic photo taken from one spot at a single point in time.  The way some take it is by pointing their camera at a mirrored ball on a tripod.  Then going to the other side of the ball and taking another photo then stiching the two together in a special (freely available on the web) program that makes a single HDRI image which appears like a warped sphere.... but when it's reflected on your object or windows in your scene looks scary-realistic.  It doesn't carry multiple times of day in the same HDR image.