Enhance the 'utc' modifier on date/time functions so that if the LHS is

already known to be in UTC, the modifier becomes a no-op.  This is not an
incompatibility because the behavior is documented as "undefined" in that
scenario.

FossilOrigin-Name: b910a3d53769689d9212a06f974ccce54844bbe4

manifest

@@ -1,5 +1,5 @@
-C Ensure\sthat\sthe\sExpr\sobjects\sthat\sdescribe\sindexed\sexpressions\sare\snot\smodified\nby\scode\sgeneration.\s\sFix\sfor\san\sassert()\sproblem\sfound\sby\sJon\sMetzman\susing\sAFL.
-D 2015-12-21T15:22:13.811
+C Enhance\sthe\s'utc'\smodifier\son\sdate/time\sfunctions\sso\sthat\sif\sthe\sLHS\sis\nalready\sknown\sto\sbe\sin\sUTC,\sthe\smodifier\sbecomes\sa\sno-op.\s\sThis\sis\snot\san\nincompatibility\sbecause\sthe\sbehavior\sis\sdocumented\sas\s"undefined"\sin\sthat\nscenario.
+D 2015-12-23T10:54:48.693
 F Makefile.in 28bcd6149e050dff35d4dcfd97e890cd387a499d
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc 5fff077fcc46de7714ed6eebb6159a4c00eab751
@@ -283,7 +283,7 @@ F src/build.c e83da4d004a4e050c01acbb821ff7a7b1019c29b
 F src/callback.c 7b44ce59674338ad48b0e84e7b72f935ea4f68b0
 F src/complete.c addcd8160b081131005d5bc2d34adf20c1c5c92f
 F src/ctime.c 60e135af364d777a9ab41c97e5e89cd224da6198
-F src/date.c fb1c99172017dcc8e237339132c91a21a0788584
+F src/date.c e4655393bb403fa310eef66cc4583d75d4d7fd93
 F src/dbstat.c ffd63fc8ba7541476ced189b95e95d7f2bc63f78
 F src/delete.c 00af9f08a15ddc5cba5962d3d3e5bf2d67b2e7da
 F src/expr.c 414f4c6b016c4494bfb11713da79f27966871d91
@@ -563,7 +563,7 @@ F test/createtab.test b5de160630b209c4b8925bdcbbaf48cc90b67fe8
 F test/cse.test 277350a26264495e86b1785f34d2d0c8600e021c
 F test/ctime.test 7bd009071e242aac4f18521581536b652b789a47
 F test/cursorhint.test 432811b62bd5ffb812729f49bba3b9ad687550bb
-F test/date.test 42973251b9429f2c41b77eb98a7b0b0ba2d3b2c0
+F test/date.test 984ac1e3e5e031386866f034006148d3972b4a65
 F test/dbstatus.test 8de104bb5606f19537d23cd553b41349b5ab1204
 F test/dbstatus2.test 10418e62b3db5dca070f0c3eef3ea13946f339c2
 F test/default.test 0cb49b1c315a0d81c81d775e407f66906a2a604d
@@ -1405,7 +1405,7 @@ F tool/vdbe_profile.tcl 246d0da094856d72d2c12efec03250d71639d19f
 F tool/warnings-clang.sh f6aa929dc20ef1f856af04a730772f59283631d4
 F tool/warnings.sh 48bd54594752d5be3337f12c72f28d2080cb630b
 F tool/win/sqlite.vsix deb315d026cc8400325c5863eef847784a219a2f
-P 8bf5e056eb8beb6e0ed5874fb24d7fe9f0b66d2b
-R e0404c0cd537e70ac5aa95195290d5e7
+P 34073ce87d88a02313217023ae92e15939192cd9
+R fb37875a5634d72cf8280c2958a6e17e
 U drh
-Z ae8c1424b866e12742c6a5c15035540f
+Z 5e4e9bf785bd86d0086510037196235b

manifest.uuid

@@ -1 +1 @@
-34073ce87d88a02313217023ae92e15939192cd9
+b910a3d53769689d9212a06f974ccce54844bbe4

src/date.c

@@ -65,6 +65,7 @@ struct DateTime {
   char validHMS;     /* True (1) if h,m,s are valid */
   char validJD;      /* True (1) if iJD is valid */
   char validTZ;      /* True (1) if tz is valid */
+  char tzSet;        /* Timezone was set explicitly */
 };
 
 
@@ -158,6 +159,7 @@ static int parseTimezone(const char *zDate, DateTime *p){
   p->tz = sgn*(nMn + nHr*60);
 zulu_time:
   while( sqlite3Isspace(*zDate) ){ zDate++; }
+  p->tzSet = 1;
   return *zDate!=0;
 }
 
@@ -590,13 +592,18 @@ static int parseModifier(sqlite3_context *pCtx, const char *zMod, DateTime *p){
       }
 #ifndef SQLITE_OMIT_LOCALTIME
       else if( strcmp(z, "utc")==0 ){
-        sqlite3_int64 c1;
-        computeJD(p);
-        c1 = localtimeOffset(p, pCtx, &rc);
-        if( rc==SQLITE_OK ){
-          p->iJD -= c1;
-          clearYMD_HMS_TZ(p);
-          p->iJD += c1 - localtimeOffset(p, pCtx, &rc);
+        if( p->tzSet==0 ){
+          sqlite3_int64 c1;
+          computeJD(p);
+          c1 = localtimeOffset(p, pCtx, &rc);
+          if( rc==SQLITE_OK ){
+            p->iJD -= c1;
+            clearYMD_HMS_TZ(p);
+            p->iJD += c1 - localtimeOffset(p, pCtx, &rc);
+          }
+          p->tzSet = 1;
+        }else{
+          rc = SQLITE_OK;
         }
       }
 #endif

test/date.test

@@ -336,6 +336,15 @@ if {$tzoffset_new==4} {
   datetest 6.8.1 {datetime('2006-04-02 02:00:00','utc')} {2006-04-02 06:00:00}
   datetest 6.8.2 {datetime('2007-03-11 02:00:00','utc')} {2007-03-11 06:00:00}
 
+  # The 'utc' modifier is a no-op if the LHS is known to already be in UTC
+  datetest 6.9.1 {datetime('2015-12-23 12:00:00','utc')} {2015-12-23 17:00:00}
+  datetest 6.9.2 {datetime('2015-12-23 12:00:00z','utc')} {2015-12-23 12:00:00}
+  datetest 6.9.3 {datetime('2015-12-23 12:00:00-03:00','utc')} \
+         {2015-12-23 15:00:00}
+  datetest 6.9.4 {datetime('2015-12-23 12:00:00','utc','utc','utc')} \
+         {2015-12-23 17:00:00}
+
+
   datetest 6.10 {datetime('2000-01-01 12:00:00','localtime')} \
       {2000-01-01 07:00:00}
   datetest 6.11 {datetime('1969-01-01 12:00:00','localtime')} \