No to JOINKEYS. It would be absurd to sort the data twice just to update one record.
Two-step process. First step takes the date-file and generates a symbol for a SYMNAMES dataset (ensure that records are 80 bytes).
Second step, which has DDnames for SYMNAMES using the file created and SYMNOUT going to SYSOUT, then a COPY operation with INREC IFTHEN=(WHEN=(logical expression) to identify the embedded date record and replace the value with the value from the symbol.
There are examples of the process if you search the forum for SYMNAMES, for instance.