Neither of these have reached me on R-devel (and only PR#8103 is on the
archive), and they seem to be the same error although neither mentions the
other. That's 'odd', to quote one of them.
"EDT" is not a valid POSIX timezone (but, say, EST5EDT is). R's docs are
quite clear that what happens with invalid inputs is system-specific.
(Windows seems often to run home to PST8PDT.) You can have timezones
without DST, but not one permanently in DST.
In short, these were user errors compounded by unhelpful error-recovery by
the OS the users were using.
As to why tm + 3600 and tm + c(0, 1) * 3600 differ: one time is in one
timezone, but two can be in different timezones so they must be handled
differently. This is handled slightly differently in the R-devel version,
which both the posting guide and the FAQ do ask you to try out (as well as
consulting the NEWS file which has a relevant entry).
Next time, please show some consideration for those who have to clear up
after you, and do as we ask in the posting guide and FAQ.