Is "difftime" an appropriate addition to .OldClassesList?

Martin Maechler 
    on Fri, 6 Mar 2026 11:36:24 +0100 writes:

Michael Chirico 
    on Thu, 5 Mar 2026 14:26:16 -0800 writes:

>> I found myself needing to add methods::setOldClass("difftime") in
    >> order to also set a coercion method for my class to difftime:

    >> methods::setAs("myClass", "difftime", ...)

    >> And thus avoid the on-load message:

    >> in method for ?coerce? with signature ?"myClass","difftime"?: no
    >> definition for class ?difftime?

    >> I see 10 or so other packages also doing the same [1].

    >> I am wondering, though, since {base} owns this class, if it shouldn't
    >> be registered already? I see inclusions for, e.g., closely-related S3
    >> classes Date and POSIXt [2].

    > In my vague recollection  "difftime" came a bit later than "Date"
    > and "POSIXlt".  This may be the reason for its non-presence.

    >> OTOH, I see .OldClassesList hasn't really been edited in 15+ years, so
    >> maybe this approach has lost favor and is better not amended anymore
    >> [3].

    > well, favors related to S4 have always been somewhat segregated ..

    >> (or maybe this is really an r-package-devel question about whether
    >> supporting as(<my class>, "difftime") is actually beneficial...)

    > The use of  as(., <...>)  is an R-help question really  not
    > restricted to package development at all.

    > I agree that "difftime" does naturally belong alongside "POSIXt"
    > and I am happy to add it to the list;
    > (I did check to see if an unneeded   setOldClass(<already_set>)
    > gives a warning, and it doesn't)

With svn rev 89582, I have committed this now to "R-devel".
Thank you, Michael, for the suggestion!

Martin