Did you read the documentation on dataset options I sent the link for? For this you need the RENAME= dataset option.
merge table1(rename=(date=refdate)) .....
Make sure to modify the references to DATE to use new REFDATE name instead in the rest of the data step.
PS Using DATE for a variable that does not actually contain DATE (number of days) values will cause you confusion later on. Your variables contain DATETIME (number of seconds) values instead of DATE values.
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