Error with BirtDateTime

When using the BirtDateTime in our reports we sometimes get the following error.

Data (id = 363):
+ A BIRT exception occurred. See next exception for more information.
Error evaluating Javascript expression. Script engine error: ReferenceError: "BirtDateTime" is not defined.
Script source: null, line: 0, text:
BirtDateTime.now()



Here is what this looks like in the report designer.
<data id="363">
<property name="marginLeft">2pt</property>
<structure name="dateTimeFormat">
<property name="category">Long Date</property>
<property name="pattern">Long Date</property>
</structure>
<property name="display">inline</property>
<list-property name="boundDataColumns">
<structure>
<property name="name">Column Binding</property>
<text-property name="displayName">Date</text-property>
<expression name="expression" type="javascript">BirtDateTime.now()</expression>
<property name="dataType">date</property>
</structure>
</list-property>
<property name="resultSetColumn">Column Binding</property>
</data>



Any ideas as to why we are getting this error sometimes with the date? When we get this error all the other fields populate correctly except the date is left blank.

Thanks for any help!