I pasted in the following chunk into the Forge window:
entity --named Customer --package ~.domain;
field string --named firstName;
field string --named lastName;
field temporal --type DATE --named birthDate;

and the result was

Java Model Exception: Java Model Status [DATE birthDate [in Customer [in [Working copy] Customer.java [in com.burrsutter.testforge1.domain [in src/main/java [in testforge1]]]]] does not exist]
at org.eclipse.jdt.internal.core.JavaElement.newNotPresentException(JavaElement.java:495)
at org.eclipse.jdt.internal.core.JavaElement.openWhenClosed(JavaElement.java:529)
at org.eclipse.jdt.internal.core.JavaElement.getElementInfo(JavaElement.java:255)
at org.eclipse.jdt.internal.core.JavaElement.getElementInfo(JavaElement.java:241)
at org.eclipse.jdt.internal.core.SourceRefElement.getSourceRange(SourceRefElement.java:218)
at org.jboss.tools.forge.ui.console.FieldPostProcessor.postProcess(FieldPostProcessor.java:36)
at org.jboss.tools.forge.ui.console.ForgeCommandProcessor$1.run(ForgeCommandProcessor.java:55)
at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:35)
at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:135)
at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4140)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3757)
at org.eclipse.ui.internal.Workbench.runEventLoop(Workbench.java:2701)
at org.eclipse.ui.internal.Workbench.runUI(Workbench.java:2665)
at org.eclipse.ui.internal.Workbench.access$4(Workbench.java:2499)
at org.eclipse.ui.internal.Workbench$7.run(Workbench.java:679)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:332)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:668)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:149)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:123)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:110)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:79)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:344)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:179)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:622)
at org.eclipse.equinox.launcher.Main.basicRun(Main.java:577)
at org.eclipse.equinox.launcher.Main.run(Main.java:1410)
at org.eclipse.equinox.launcher.Main.main(Main.java:1386)