private TimeStampFunctionNode toTime(GroupingExpression arg, TimeStampFunctionNode.TimePart timePart) {
        if (timeOffset == 0) {
            return new TimeStampFunctionNode(toExpressionNode(arg), timePart, true);
        }
        AddFunctionNode exp = new AddFunctionNode();
        exp.addArg(toExpressionNode(arg));
        exp.addArg(new ConstantNode(new IntegerResultNode(timeOffset)));
        return new TimeStampFunctionNode(exp, timePart, true);
    }
Converts the given ast type grouping expression to a corresponding back-end type expression.

@param exp The expression to convert.
@return The corresponding back-end expression.
@throws IllegalInputException Thrown if the given expression could not be converted.
container-search/src/main/java/com/yahoo/search/grouping/vespa/ExpressionConverter.java