Thanks, I think I found an solution in the docs you referenced for _tzset:

Use the following syntax to set the TZ environment variable:

set TZ=tzn[+ | –]hh[:mm[:ss] ][dzn]

tzn
Three-letter time-zone name, such as PST. You must specify the correct
+ offset from local time to UTC.
[download]

So I tried "EST5EDT" instead of -0400, and that gives the right result. I'm still not sure why it behaves differently when called via system() though.

update:used code tags so brackets in syntax aren't hidden.