ssconvert assumes that Excel/xlsx dates are in US locale?

[EML <sa212+gnumeric cyconix com>]

My input Excel spreadsheets have dates in DD/MM/YYYY format, so today 
would be 14/05/2022 (this is normal in the UK; no-one is going enter a 
date as 2022-05-14). I convert these to csv with:

ssconvert --export-type=Gnumeric_stf:stf_assistant -O 'format=preserve 
quoting-mode=auto separator=,' file.xlsx file.csv

With 'preserve', the resulting csv shows 05-14-22, so it's assuming a US 
locale. Is there any way I can change this? The only mention of 'locale' 
in the man page refers to export, not import. Thanks.