Use the proper informat and the INPUT function. There are several informats that might work including B8601DJ
data want;
set test;
num_date_time=input(date,b8601dj12.);
format num_date_time datetime16.;
run;
Hint: Always use built in SAS functions (in this case INPUT with the proper informat) to work with dates and date/times. Don't try to create your own.
Hint 2: Date/time values should not have a variable name of DATE which indicates it is a date value and not a date time value.
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