[date_time] try to show zone offset where appropriate

Related to #703

src/base/date_time_scanner.cc

@@ -313,13 +313,13 @@ date_time_scanner::to_localtime(time_t t, exttm& tm_out)
         tm_out.et_tm.tm_isdst = 0;
 
         new_gmt = tm2sec(&tm_out.et_tm);
-        this->dts_local_offset_cache = t - new_gmt;
+        this->dts_local_offset_cache = new_gmt - t;
         this->dts_local_offset_valid = t;
         this->dts_local_offset_expiry = t + (EXPIRE_TIME - 1);
         this->dts_local_offset_expiry
             -= this->dts_local_offset_expiry % EXPIRE_TIME;
     } else {
-        time_t adjust_gmt = t - this->dts_local_offset_cache;
+        time_t adjust_gmt = t + this->dts_local_offset_cache;
         gmtime_r(&adjust_gmt, &tm_out.et_tm);
     }
     tm_out.et_gmtoff = 0;

src/logfile_sub_source.cc

@@ -336,6 +336,8 @@ logfile_sub_source::text_value_for_line(textview_curses& tc,
                 {
                     adjusted_tm.et_flags &= ~ETF_Z_IS_UTC;
                 }
+                adjusted_tm.et_gmtoff
+                    = format->lf_date_time.dts_local_offset_cache;
                 len = format->lf_date_time.ftime(
                     buffer,
                     sizeof(buffer),

src/ptimec.hh

@@ -981,7 +981,7 @@ ftime_z(char* dst, off_t& off_inout, ssize_t len, const struct exttm& tm)
         return;
     }
 
-    if (tm.et_flags & ETF_Z_FOR_UTC) {
+    if (tm.et_gmtoff == 0 && tm.et_flags & ETF_Z_FOR_UTC) {
         PTIME_APPEND('Z');
         return;
     }

test/expected/test_logfile.sh_82d65f4dac07b1623f8202444d5b9144096e8243.out

@@ -0,0 +1,3 @@
+192.168.202.254 - - [20/Jul/2009:15:59:26 -0700] "GET /vmw/cgi/tramp HTTP/1.0" 200 134 "-" "gPXE/0.9.7"
+192.168.202.254 - - [20/Jul/2009:15:59:29 -0700] "GET /vmw/vSphere/default/vmkboot.gz HTTP/1.0" 404 46210 "-" "gPXE/0.9.7"
+192.168.202.254 - - [20/Jul/2009:15:59:29 -0700] "GET /vmw/vSphere/default/vmkernel.gz HTTP/1.0" 200 78929 "-" "gPXE/0.9.7"

test/test_logfile.sh

@@ -399,6 +399,8 @@ EOF
 
 run_cap_test env TZ=America/Los_Angeles ./drive_logfile -t -f access_log ${srcdir}/logfile_access_log.0
 
+run_cap_test env TZ=America/Los_Angeles ${lnav_test} -n ${srcdir}/logfile_access_log.0
+
 run_test ./drive_logfile -t -f generic_log ${srcdir}/logfile_tai64n.0
 
 check_output "tai64n timestamps interpreted incorrectly?" <<EOF