If you have a wrongly select date for the newly hired employee, you can update it in People:
Step 1 - Go to People > Directory > Manage employees
Step 2 - Look for the employee's name
Step 3 - Click Update onboarding date
Step 4 - Select new onboarding date
Step 5 - Click Update
You can change the onboarding date to the future date only. Should you need to invite the newly hired employee earlier than it was, make sure the new invite date is tomorrow onwards.