There is an outstanding application for long range flight and cabin crews to compute rest periods between the crew. It is derived from an original Android app, and the author has just recently had it posted in App World. It is in the Utilities/Calculators section, and is titled "ZZTrop", meaning zzzzzz's in the troposphere. I highly recommend it to any and all flight crews carrying the Playbook flag, doing mighty battle against the I-crowd.