How to use null_flavor in the flat format?

Thanks Ian.
And what do you do when you have to insert a date (mandatory in the template) and you don’t have it and can’t retrieve it ? do you make a guess based on the dates (i f any) available in your input data or you put always the same date (e.g. a date in the distant past or future) that you treat as fake when you re-read the data?