Perhaps Nintex is converting those conditions to CAML, and in this special case (datetime fields) it's needed to use the special attribute named IncludeTimeValue, that perhaps was ignored on the internal CAML conversion.

Usually if you write a CAML like this should be as the following sample.

<where><geq><fieldref Name='CreateDate'></fieldref><value Type='DateTime' IncludeTimeValue='TRUE'>19.06.2014 12:00:00</value></geq></where>

It seems to be case for support[at]nintex.com (Support​ )