Jim,
To do this I would have to add another field - time, the multi valued fields are half hourly time periods e.f.

Key, Date, hh1, hh2 ... hh47, hh48

Due to the nature of the information I would end up with around 23 million rows if I held the data in the format

A00408, 01/01/2000, 00:00, 20
A00408, 01/02/2000, 00:30, 21
A00408, 01/02/2000, 01:00, 22

I was hoping I could retrive the maximum value for each half hour and the time that this occured for each month of the year from the way the table is structured now. The data is pulled from an Oracle database through a Remote View and the appended to a local DBF for manipulation - check number of days with data, days with zeros populated.
The time to convert from this format into the one you describe would be time consuming.

Thankyou for your suggestions.