emacs time, timezone don't match system after upgrade

[Tom Roche]

summary: After updating cygwin to 1.7.1-1, which updated my emacs to
23.1, I now see 

(current-time-string)
"Mon Dec 28 01:21:44 2009"
(current-time-zone)
(-3600 "GMT")

when bash says

$ date
Sun Dec 27 21:21:44 EST 2009

How to fix?

details:

I've been running emacs on cygwin for a long time, until yesterday (27
Dec 09) emacs=22.3 on cygwin=1.5.25 (both on winxp sp3). Today I
updated my cygwin to the new cygwin 1.7.1, which delightfully updated
my emacs to

GNU Emacs 23.1.1 (i686-pc-cygwin, X toolkit, Xaw3d scroll bars)
 of 2009-07-30 on laptop

Mostly this "just works," with no changes required to my all-too-huge
.emacs. However today I'm noticing that the time in my emacs mode line
is now 4 hours ahead of my system time. 

(current-time-string)
"Mon Dec 28 01:21:44 2009"
(current-time-zone)
(-3600 "GMT")

when bash says

$ date
Sun Dec 27 21:21:44 EST 2009

The latter matches my system time (which is backed by NTP, and matches
that of other nearby boxes into which I can shell) and timezone. Note
also that neither $TZ (in cygwin) or %TZ% (in winxp) are set.

How can I make my emacs time and timezone match my system? Feel free
to reply directly to me as well as to the list.

TIA, Tom Roche <Tom_Roche@pobox.com>