Jacques, Instead of modifying the groovy files, try specifying the data type in the screen widget.
Example in ReportFinancialSummaryScreens.xml:
<set field="fromDate" from-field="parameters.fromDate" type="Timestamp"/>
<set field="thruDate" from-field="parameters.thruDate" type="Timestamp"/>
<script
location="component://accounting/webapp/accounting/WEB-INF/actions/reports/TransactionTotals.groovy"/>
-Adrian
