I could be wrong here, but there usually is two actual time of lift-off, and it all depends on who you ask.<br /><br />The first lift-off time is the time at which the shuttle computers declare the lift-off time at command (usually the time at which the hold-down bolts are commanded blown) and the second is the actual physical time at which the SRB's leave the bolts on the pad.<br /><br />The computers guys use the first, while the rest of the real world use the later time of physical lift-off.<br /><br />Again, I could be wrong...