system time is 5 hours off in Windows

I've found the warning in the documentation for as.POSIXct to try setting
TZ on UNIX-like systems if the POSIX functions give unusual results;
perhaps this should apply to Windows systems, too?  That doesn't explain

the return of date() which relies on the OS to set the time zone (and, when
I execute the same code as a stand-alone program, I get the correct system
time).  It is as if some startup code looks for TZ in the environment, and
if it is not there, sets it to a value which, perhaps, the system does not
recognize as valid, and defaults to GMT.

This is what I get inside Rgui when I query the TZ variable (I have not set
it in the environment):

Sys.getenv("TZ")

                     TZ
"Eastern Daylight Time"

It seems to me that, if this is going to be used to set the time zone for
POSIX functions, it should look like one of the standard ways of specifying
a time zone (all I know about this is what I read in the libc manual about
the POSIX date-time functions, so I may be way off base here for the
Windows implementation).