1. Before entering any tasks, you have to specify how many work hours equal a workday by using option Hours per Day on the tab Schedule (ribbon File, item Options). Microsoft Project uses this setting to convert between time units; it is the time unit conversion factor. All durations will be recalculated when you change this option. For example, if Hours per Day is set to eight hours and you enter a duration of five days, MS Project knows this equals 8 * 5 = 40 hours. If you then change Hours per Day to seven, MS Project changes this duration to 5.71 days (= 5 * 8 / 7). If you start with the wrong conversion factor, MS Project will interpret durations you enter incorrectly, which may result in a schedule that is too low or high in its forecast.
2. There is a workaround to keep the current durations without having to re-enter all durations when you change the Hours per Day setting. Before changing Hours per Day, copy all durations to one of the extra text fields (Text1, for example), then change Hours per Day. Make sure you have task field Type set to Fixed Units for all tasks to ensure that MS Project does not change resource assignments. Then copy the durations from Text1 back into the Duration field.
3. MS Project sets constraints on each recurring detail task when they are Auto Scheduled. The constraints will keep them on their dates in the timescale. Although we recommend against using constraints in schedules, there is nothing wrong with constraints on recurring detail tasks. They are a legitimate exception to this rule. Recurring detail tasks are typically not hooked up into the network of dependencies and scheduling constraints will keep them on the proper dates.
This article originally appeared as content in the textbook Forecast Scheduling with Microsoft Project 2013: Best Practices for Real World Projects, by Eric Uyttewaal, PMP, MVP Project.