@Nullable @Override public String toDruidExpression(RexNode rexNode, RelDataType rowType, DruidQuery query
    ) {
      final RexCall call = (RexCall) rexNode;
      if (call.getOperands().size() != 1) {
        throw new IllegalStateException("to_date() requires 1 argument, got " + call.getOperands().size());
      }
      final String arg = DruidExpressions.toDruidExpression(call.getOperands().get(0), rowType, query);
      if (arg == null) {
        return null;
      }
      return DruidExpressions.applyTimestampFloor(
          arg,
          Period.days(1).toString(),
          "",
          timezoneId(query, call.getOperands().get(0)));
    }
Expression operator conversion form Hive TO_DATE operator to Druid Date cast.
ql/src/java/org/apache/hadoop/hive/ql/parse/DruidSqlOperatorConverter.java