You need to do a calculation. A datetime is the number of seconds from Jan 1, 1960, while a date variable is the number of days.
To do the conversion you can use the datepart() function in a proc sql or data step, but not in proc datasets or via an alter table.
If you just want to change the display format of your data you can use DTDATE9. or a format in that family. Unfortunately I don't think there's one that matches the format you want though you can always create your own if you'd like, see Picture formats.
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