When did Pitcairn switch its time zone?

[Paul Eggert]

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.