Fix gmtime and localtime on negative timestamps with fractional seconds (#3634)

The whole seconds were truncated toward zero while the fractional part
was measured from the floor, so any negative non-integral timestamp was
rendered one second late; gmtime was not monotonic across the epoch.

src/builtin.c

@@ -1757,7 +1757,7 @@ static jv f_gmtime(jq_state *jq, jv a) {
   struct tm tm, *tmp;
   memset(&tm, 0, sizeof(tm));
   double fsecs = jv_number_value(a);
-  time_t secs = fsecs;
+  time_t secs = floor(fsecs);
   jv_free(a);
   tmp = gmtime_r(&secs, &tm);
   if (tmp == NULL)
@@ -1771,7 +1771,7 @@ static jv f_gmtime(jq_state *jq, jv a) {
   struct tm tm, *tmp;
   memset(&tm, 0, sizeof(tm));
   double fsecs = jv_number_value(a);
-  time_t secs = fsecs;
+  time_t secs = floor(fsecs);
   jv_free(a);
   tmp = gmtime(&secs);
   if (tmp == NULL)
@@ -1792,7 +1792,7 @@ static jv f_localtime(jq_state *jq, jv a) {
   struct tm tm, *tmp;
   memset(&tm, 0, sizeof(tm));
   double fsecs = jv_number_value(a);
-  time_t secs = fsecs;
+  time_t secs = floor(fsecs);
   jv_free(a);
   tmp = localtime_r(&secs, &tm);
   if (tmp == NULL)
@@ -1806,7 +1806,7 @@ static jv f_localtime(jq_state *jq, jv a) {
   struct tm tm, *tmp;
   memset(&tm, 0, sizeof(tm));
   double fsecs = jv_number_value(a);
-  time_t secs = fsecs;
+  time_t secs = floor(fsecs);
   jv_free(a);
   tmp = localtime(&secs);
   if (tmp == NULL)

tests/optional.test

@@ -10,3 +10,11 @@ strftime("%A, %B %e, %Y")
 1435677542.822351
 "Tuesday, June 30, 2015"
 
+# Timestamps before the epoch are not supported on mingw/WIN32
+gmtime
+-1.25
+[1969,11,31,23,59,58.75,3,364]
+
+map(gmtime) | . == sort
+[-1.5,-1,-0.5,0,0.5,1,1.5]
+true