I this example, the customer was confused when changing the dates for a task that it moved the start date back a day, yet for exactly the same resource it moved the finish forward a day? What was going on? The help understand this I'll give some background first, and it is to do with the time zones of the project and of the resource. The project is set to use PST (GMT-8) and to also respect the time zone of any assigned resources - and in this case our resource, Henrietta Mueller works in GMT+5. I also happen to be running under en-GB - so my dates are showing as dd/MM.
Before any assignment is made the tasks have a 9am start time, and a 5pm end time:
Once I assign Henrietta, then the earliest they can start is 9pm PST (21:00), which is 9am in Henrietta's time zone:
If I now make a change to the start date or end date, what will happen? If I change the start date of Task 1 to August 3rd, I get a message saying the start date was move to the NEXT working day, which would be August 4th - yet the date is actually set as August 6th. Why?
To understand what happened we need to remember that there are two things at play here - the task and the assignment. When we change the date, we are doing this to the task, yet the date, and more importantly the time, is being driven by the assignment. So when we change the date to August 3rd, the start time is 9pm, or 21:00. This is after the project day ends, at 5pm, so it goes back to the next day. As luck would have it (or my convenient choice of which dates to choose), the next day is Friday 4th, but for Henrietta, their Friday has already finished, the next working day for her appears as August 6th on the project plan, at 9pm, but is their Monday morning.
What happens when we change the finish date? In this case I'll select Wednesday 2nd August as the next finish - and I get a pop-up that the task finish was moved to the previous working day:
The logic here is very similar. The time element of the finish is 5am, the end of Henrietta's working day. As this is before the project day starts in the PST time zone, then this is pushed back to the previous day - then actually starts at 9pm on August 1st.
The logic for both of these is the same; the assignment times are being used to find the new task start/finish, then this sets the day, then the assignment is moved which may also change the day if working time isn't available in the day where the task now lands.
In both of these cases a constraint is added to the task which can be seen as a star in the corner of the calendar - then opening the calendar will detail the precise constraint. You can also remove the constraint if you need to. Please note that constraints also get added if you move a task start or finish in the timeline view by dragging the tasks.
If you are driving the schedule using the project start date, and then using the dependencies between the tasks then you may find that you don't really want constraints. Also taking a look at the Assignments tab may help you understand where the work is being carried out.