Creating a calculated field similar to the following should give you the desired result:
if int(mid(str([Datetime]), 14, 1))<5 then datetime(left(str([Datetime]), 13)+ "5:00" )
elseif int(mid(str([Datetime]), 14, 1))>5 then datetime(left(str([Datetime]), 12)+(str(int(mid(str([Datetime]), 13, 1))+ 1)+ "0:00" ))
Hope this helps!
The way I prefer to do this is to convert the datetime to a floating point number, truncate it to the interval size I want and convert it back again. With this approach you can easily parameterise the interval size. I've written this up showing how to parameterise it for the soon to be launched (or relaunched) Tableau Calculation Reference Library - but here's a sneak preview:
DATETIME(INT(FLOAT([datetime]) * 288) / 288)
288 is the number of 5 minute intervals in a day (24 * 12).