After upgrading ArcInfo Workstation, the AML [DATEFORMAT] function in the TESTDATE.AML returns the error:
"Invalid input string"
The previous ArcInfo Workstation install was customized to deal with the operating system date time format in a particular way. That customization was lost upon upgrade.
The [FORMATDATE] documentation in ArcInfo Workstation help provides more information on why and how to update this file.