As of NiFi 1.10 (via
NIFI-5254), the version of Groovy included with NiFi is 2.5.0. There was a breaking change in that version where the date utilities (such as GDK java.util.Date.parse()) were moved into a separate module that is not included by default with the groovy-all module.
This module is now missing from the NiFi Groovy components and thus any script using those classes/methods will fail: http://groovy-lang.org/releasenotes/groovy-2.5.html#Groovy2.5releasenotes-Breakingchanges
The dateutil (and possibly other) Groovy modules should be included as dependencies in the groovy bundles (nifi-groovyx-bundle, nifi-scripting-bundle, possibly the groovy plugin for unit tests, etc.)