>>If I understand correctly, you want to print headers or footers every time an expression changes?
>>
>>Give the command "Report | Data Grouping", and type an expression which should be checked, whether it changes. For example, for a month, this might be: left(dtos(MyDate),6)).
>>
>>This will create two new report bands, for this group. You can put report fields into these bands, to print the desired information each time a month changes.
>>
>>>

>>>January         1 2     5 6 7 8 9    1213141516    1920212223    2627282930
>>>February  2 3 4 5 6     910111213    1617181920    2324252627
>>>

>
>I want to print /different/ headers every time the expression changes.
>
>The monthname was easy and I've got code to produce the header, but it's too complex for the Report Writer's expression builder. How do I pre-produce the date string and get the RW to use the correct one at the correct time?
>
>The footer problem solved itself.

I would suspect you need some UDF, which you call form the report. This UDF would use a loop to generate the desired string.