Why not both. You can simply divide up a 24/7 schedule on 3 shifts and have open slots which officers can trade back and forth. Offer everyone a default schedule of 3 shifts of 8,8,8,8,8,off,off each week, shifting the work week ahead by 1 day each week until they're back to the original one 7 weeks later. Allow them to offer up or accept 4 hour blocks of time on a temporary or permanent basis, so some officers could take 12 hour days if they want. Many people won't necessarily want to work a 12