If an employee has left your organisation and returns at a later date, you can reactive their SMB account providing the account was not deleted by you.
To reinstate a leaver, click on the "Employees" option on the main menu, then click on the "View Ex-Employees" link at the top of the page. Select the account you wish to reinstate, then use the link in the "Employee Has Left" panel at the top of the page to reactivate their account.
If you wish, you can simply reinstate their existing employment by clicking on the link to "Reactivate the account" or, if they are restarting with you in a new role, click on the "Click here to setup their new role" link to create their new position before restarting their account. This will restart all allocations while retaining their existing job history.
If you have restarted the employee's employment without creating a new job record, then their job history will run from their original start date. If you want to restart their employment with a new role and did not select that option when restarting their record, you can achieve the same result by manually setting up their new role. To do this, go to their record from the "Employees" option on the main menu and then click on the "Job History" item on the large employee menu,
When the Job History page is shown, click on the "Add New Job Role" link at the top of the page and enter the details for the new role, including the start date, on the form shown. Then, select the "Is Current Role" and "Reset Allocation" checkboxes before submitting the form. This will then close off their previous record and enter a new start date on their employment record, as shown on the employee dashboard.