Are you sure you aren't thinking about a contractual requirement? Didn't 12 day trips exist pre-117?

The old rules required 24 hours free from duty in the last 7 days. 117 requires 30 hours free from duty in the last 168 hours (7 days). During the rule making, I remember the FAA saying they wanted to up the 24 hours to 36 hours initially, but backed off to 30 hours for the final rule, saying that a 36 hours off requirement may do more harm than good in terms of our schedules and the airlines scheduling ability. The hope was that 30 hours offered a better chance at a full calendar day off in the stretch, where 24 hours didn't.

117 requires the 30 hours to be prospectively given, where I'm not 100% sure if the old 24 hours had to be. Whether it did or not, 117 doesn't allow for a 24 hour layover to turn into a 30+ hour layover if a cancellation occurs at hour 20. The only way for the 30 hour rest to be reset in 117, is for the 30 hours to be prospectively known completely in advance. So a 24 hour layover turning into a 44 hour layover, at hour 20, would not satisfy a 30 hour rest reset, unless the pilot had 30 hours off from the 20 hour mark and the layover turned into a 50 hour layover at hour 20.

One follow up note, is the pilot is off duty until the 24 hour mark and may not find out about the cancellation until reporting at the 24 hour mark or checking their schedule prior to leaving the hotel. In that case, their layover would have to become at least 54 hours to satisfy a 30 hour rest reset in 117.

http://www3.alpa.org/portals/alpa/fa...-FTDT-Rule.pdf

Federal Register/ Vol. 77, No. 2 / Wednesday, January 4, 2012 / Rules and Regulations
11. Rest
Carriers will be required to provide their crew with a 10-hour rest opportunity prior to commencing a duty period that includes flying. While the 10-hour rest period may include the amount of time it takes to get to or from a flightcrew member’s house or hotel room, the actual amount of time required for a sleep opportunity may not be reduced below 8 hours. In addition, the length of continuous time off during a 7-day period has been extended from 24 hours under the existing rules to 30 hours. Additional time off is required for individuals whose internal clock may be off because of flipping back and forth between different time zones