Get returning value of a function in Inno setup

i need to write in registry the current date, when my app were installed. For this i have created function in [code] section and trying to call it from [registry] section but it doesnt work. How can i call my function from [registry] section to get the value which it returns?

[Registry]
Root: HKLM; Subkey: SOFTWARE\MyAppName; ValueType: string; ValueName: date; ValueData: **{code:DateTime}**; Flags: createvalueifdoesntexist; Tasks: ; Languages: 
[Code]
function DateTime() : String;
begin
  result := GetDateTimeString('dd/mm/yyyy hh:nn:ss', '-', ':');
end;