Invalid date passed (&)

What causes this issue?

The date must be transmitted in the format "YYYYMMDD" and is then
checked by the system with respect to its validity.

System Response

Conversion is not possible.

How to fix this error?

Correct the application program, in particular by entering the date in
a suitable field. This way the system performs an automatic check.

Error message extract from SAP system. Copyright SAP SE.