When setting up assignments, the date picker always uses the moodle system time as a starting point. This might be convenient when the assignment is being set up the instant it is available but is rarely satisfactory if the course prep time is nowhere near the course start time. Course Content developers should have better tools for assignment availability and due date settings. For courses that have a typical weekly schedule, it would help if some sort of date math was available so dates didn't have to be changed manually from semester to semester.
Other possibilities include reading a time date settings from a separate browser window or from a local text file.