Currently the timedelta fields are exported as HH:MM (seconds are included only when non empty) but this format is not managed by libreoffice to perform operations (i.e sum).
I think it will be good to export the format using the expected time format for the spreedsheet. Do you think this will be a good addition?
I think even if the spreadsheet program does not auto-detect it as time, you can apply manually a time format on the column.
So for me we do not need to adapt as our format is quite standard.