As a first step, let me document this for the POSIX functions.
2017-04-30 Bruno Haible <br...@clisp.org>
Document the problem with the Cygwin environment variable TZ.
* doc/posix-functions/tzset.texi: Add note about TZ.
* doc/posix-functions/ctime.texi: Likewise.
* doc/posix-functions/localtime.texi: Likewise.
* doc/posix-functions/mktime.texi: Likewise.
* doc/posix-functions/strftime.texi: Likewise.
* doc/posix-functions/wcsftime.texi: Likewise.
* doc/pastposix-functions/ftime.texi: Likewise.
diff --git a/doc/pastposix-functions/ftime.texi
b/doc/pastposix-functions/ftime.texi
index 0fcaed1..582a088 100644
--- a/doc/pastposix-functions/ftime.texi
+++ b/doc/pastposix-functions/ftime.texi
@@ -16,6 +16,9 @@ Portability problems not fixed by Gnulib:
This function is missing on some platforms:
Mac OS X 10.5, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, IRIX 5.3, Solaris 2.4.
@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
+@item
This function is marked as ``legacy'' in POSIX. Better use @code{gettimeofday}
or @code{clock_gettime} instead, and use @code{ftime} only as a fallback for
portability to Windows platforms.
diff --git a/doc/posix-functions/ctime.texi b/doc/posix-functions/ctime.texi
index f5a7c27..e54a7b5 100644
--- a/doc/posix-functions/ctime.texi
+++ b/doc/posix-functions/ctime.texi
@@ -13,6 +13,9 @@ Portability problems fixed by Gnulib:
Portability problems not fixed by Gnulib:
@itemize
@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
+@item
This function may overflow its internal buffer if an invalid year is passed.
@item
The @code{ctime} function need not be reentrant, and consequently is
diff --git a/doc/posix-functions/localtime.texi
b/doc/posix-functions/localtime.texi
index 49a6ddd..1d6acdb 100644
--- a/doc/posix-functions/localtime.texi
+++ b/doc/posix-functions/localtime.texi
@@ -12,7 +12,11 @@ Portability problems fixed by Gnulib:
Portability problems not fixed by Gnulib:
@itemize
-@item On some platforms, this function returns nonsense values for
+@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
+@item
+On some platforms, this function returns nonsense values for
unsupported arguments (like @math{2^56}), rather than failing:
FreeBSD 10.
@end itemize
diff --git a/doc/posix-functions/mktime.texi b/doc/posix-functions/mktime.texi
index 9756948..ffb7b79 100644
--- a/doc/posix-functions/mktime.texi
+++ b/doc/posix-functions/mktime.texi
@@ -16,4 +16,7 @@ Portability problems fixed by Gnulib:
Portability problems not fixed by Gnulib:
@itemize
+@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
@end itemize
diff --git a/doc/posix-functions/strftime.texi
b/doc/posix-functions/strftime.texi
index e58e1ef..d371818 100644
--- a/doc/posix-functions/strftime.texi
+++ b/doc/posix-functions/strftime.texi
@@ -13,6 +13,9 @@ Portability problems fixed by Gnulib:
Portability problems not fixed by Gnulib:
@itemize
@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
+@item
The Windows C runtime library (which is used by MinGW) does not
support the %e specifier (and possibly the other more recent SUS
specifiers too, i.e., %C, %D, %h, %n, %r, %R, %t, and %T).
diff --git a/doc/posix-functions/tzset.texi b/doc/posix-functions/tzset.texi
index 35fc509..30b147c 100644
--- a/doc/posix-functions/tzset.texi
+++ b/doc/posix-functions/tzset.texi
@@ -16,4 +16,7 @@ Solaris 2.6.
Portability problems not fixed by Gnulib:
@itemize
+@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
@end itemize
diff --git a/doc/posix-functions/wcsftime.texi
b/doc/posix-functions/wcsftime.texi
index 0faa211..8ab82c2 100644
--- a/doc/posix-functions/wcsftime.texi
+++ b/doc/posix-functions/wcsftime.texi
@@ -16,6 +16,9 @@ Portability problems not fixed by Gnulib:
This function is missing on some platforms:
OpenBSD 3.8, Minix 3.1.8, IRIX 5.3, Solaris 2.5.1, Cygwin 1.5.x, BeOS.
@item
+On native Windows platforms (mingw, MSVC), this function works incorrectly
+when the environment variable @code{TZ} has been set by Cygwin.
+@item
On AIX and Windows platforms, @code{wchar_t} is a 16-bit type and therefore
cannot
accommodate all Unicode characters.
@end itemize
