You must enter their new hire date into QBO. If you don't, their ROE's won't be correct if and when they require an ROE in the future. Having said that, it is a good practice to record original hire dates and lay-off dates, etc. just so you know the history of each employee's comings and goings.
In Quickbooks Desktop I create custom fields for that right in the Employee's payroll profile, which is very convenient because I can pull reports with that data. Unfortunately, I see no way to do this in QBO, but I could be wrong. You may just have to track that in a separate spreadsheet.