I have a piece of software that can generate monthly summary data from
WL archive files. This includes a feature to set the station day to
start from whichever hour you choose.

This software is unfortunately in permanent beta at present, while I
try to make time to properly complete a first full release (while at
the same time keep adding features). As a result, it's currently still
a work in progress, with potential bugs and lack of polish. But
contact me if this is of any interest.

John Dann
www.weatherstations.co.uk