In this case you need to choose 'replace existing calendar' option in the importer or import into project with working weekends In the original MS Project file it starts on weekend day and you import it into a project with non-working weekends.So, why a task may change the dates after import: We recommend to start from the earliest task in the schedule because it might turn out to be the cause of changes in the whole subsequent chain.
If you see a warning saying that some of the tasks changed their dates, you may use the list below as a guide to fixing this issue. The reason is that Microsoft Project and GanttProject are different applications with different features and different behavior. While we do our best to import MS Project files as is, you may notice that after import some of the tasks change their dates. This page describes possible reasons of task dates change after import from MS Project file.
