When did Pitcairn switch its time zone?

A public-domain time zone database <ftp://elsie.nci.nih.gov/pub/> maintained at the US National Institutes of Health attempts to keep track of all the time zones in the world, along with the history of exactly when each location changed from one time zone to another. Currently it lists Pitcairn as consistently being 8.5 hours ahead of Greenwich since 1901, but Mark Brader recently reported that the 1997 UK Hydrographic Office time zone map lists Pitcairn as being 9 hours ahead of Greenwich, and the 1997 CIA time zone map agrees. Is this correct? Do you happen to know when the switch occurred (ideally, the date and time), or know who might know? Any info that you can give would be helpful in correcting the time zone database. Thanks.