There doesn’t seem to be such a field in any of the user tables. All of our users are tied to employee records (which also don’t have created dates), but those are tied to payroll records which do have hire dates. That’s probably your best bet for historic data.
We also use single-sign on, which is tied to Exchange records. I’m not an Exchange admin, so I can’t tell you from personal experience if it tracks create dates. Prof. Google says it does, so there’s that.
If neither of those options work for you, I can’t think of much else that would provide good backwards visibility.
In terms of forward visibility, I’d use a BPM designed to either log new records, or fill in a custom field attached to erp.UserFile, which you could surface in a form customization. Though, I’d make sure that field is un-editable.