Not sure I fully understand your question. A packaged (.twbx) workbook would be helpful. So Your Question Didn't Get Answered...
That said, I think you could convert your minute time stamps to their corresponding half hours sort of like a DATETRUNC function would do...
+STR(YEAR([DateTimeField]))+' ' //year
+IF DATEPART('minute',[DateTimeField]) > 30 THEN '30' ELSE '00' END //minute
+ ':00' //second
Then, perform your join on this calculated field?