Postgres returns a timestamp from date_trunc, whereas MySQL returns a

date from date().  We should probably just modify DateFuncs.date to
return a date in both cases (perhaps with a separate DateFun.DateCast
class), but for now it's easier to have MySQL return a timestamp as
well.

src/java/com/samskivert/depot/impl/MySQLBuilder.java

@@ -90,7 +90,10 @@ public class MySQLBuilder
         public Void visit (DateTruncate exp)
         {
             // exp.getTruncation() is currently always DAY
-            return appendFunctionCall("date", exp.getArg());
+            _builder.append(" cast(");
+            appendFunctionCall("date", exp.getArg());
+            _builder.append(" as datetime)");
+            return null;
         }
 
         protected String getDateFunction (Part part)