If you have a wrongly selected a 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 a 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.