Completion date is important date if you are considering 485 Graduate visa after your study in Australia.
You can apply 485 visa after your completion date and within 6 months of the date of course completion. Completion date is not the date of conferral, at a graduation ceremony.
You should not apply 485 on the same day as your completion date! Please check following case, to see what happens if you apply 485 on the same day as completion date.
http://www.austlii.edu.au/au/cases/cth/AATA/2017/1204.html
If you have any concerns with your 485 visa application, please contact Storyoz now.