How to achieve this in an extension?

[Soham_Shah]

This is my code

Calendar instant = (Calendar) GetProperty(date,"Instant");
String instants = String.valueOf(Dates.Year(instant));
SetProperty(title,"Text",String.join(Dates.MonthName(instant) , " " , (instants)));

It gets compiled but it shows me error when i use the extension

This is what i want extension should set text to

This is the error i face