Depending on your Organization configuration, Creating (or Modifying) an assignment will force Shift Start/End Times to match the Post Start/End Time resulting in Error messages upon Saving.
Currently, the workaround is to expose and adjust the Shift Start/End Time making sure the correct time is selected.
The Indeavor Technology Team is working diligently to address these issues at the highest priority. Code changes to accommodate this problem should be released within the next couple of weeks.
Thank you for your patience; we look forward to communicating a long-term solution shortly.