tz>0 to select + or "" in dates doesn't work if you're actually in GMT.
a nearby Spam filter decided messages were spam because the
date wasn't RFC822 format.  i wonder who writes such stuff.
anyhow, probably should change it to tz>=0

i couldn't find any other instances

Reference: /n/sources/patch/applied/gmt-rfc822
Date: Thu Feb  3 13:55:44 CET 2005

--- /sys/src/cmd/upas/marshal/marshal.c	Thu Feb  3 13:55:44 2005
+++ /sys/src/cmd/upas/marshal/marshal.c	Thu Feb  3 13:55:44 2005
@@ -733,7 +733,7 @@
 
 	return Bprint(b, "Date: %s, %d %s %d %2.2d:%2.2d:%2.2d %s%.4d\n",
 		ascwday[tm->wday], tm->mday, ascmon[tm->mon], 1900+tm->year,
-		tm->hour, tm->min, tm->sec, tz>0?"+":"", tz);
+		tm->hour, tm->min, tm->sec, tz>=0?"+":"", tz);
 }
 
 int
@@ -935,7 +935,7 @@
 	return fprint(fd, "From %s %s %s %d %2.2d:%2.2d:%2.2d %s%.4d %d\n",
 		user,
 		ascwday[tm->wday], ascmon[tm->mon], tm->mday,
-		tm->hour, tm->min, tm->sec, tz>0?"+":"", tz, 1900+tm->year);
+		tm->hour, tm->min, tm->sec, tz>=0?"+":"", tz, 1900+tm->year);
 }
 
 char *specialfile[] =