Use alternate test for DST change

As AlexDaniel pointed out, what if there is more than an hour between
calls?  Fixed by taking the absolute hour value since epoch and compare
against that.

src/core.c/DateTime.pm6

@@ -7,7 +7,7 @@ my class DateTime does Dateish {
       # $dt.utc.local.utc is equivalent to $dt.utc. Otherwise,
       # DST-induced ambiguity could ruin our day.
 
-    my int $last-minute-seen = 60;
+    my int $last-hour-seen;
     my int $TZ-was-set-explicitly;
     my int $TZ-offset;
     sub get-local-timezone-offset() {
@@ -17,12 +17,13 @@ my class DateTime does Dateish {
 
         # not set explicitly
         else {
-            my int $utc = nqp::time_i;
-            my $lt := nqp::decodelocaltime($utc);
+            my int $utc  = nqp::time_i;
+            my int $hour = nqp::div_i($utc,3600);
 
             # first time, or possible DST change
-            if nqp::atpos_i($lt,1) < $last-minute-seen {
-                $last-minute-seen = nqp::atpos_i($lt,1);
+            if nqp::isne_i($hour,$last-hour-seen) {
+                my $lt := nqp::decodelocaltime($utc);
+                $last-hour-seen = $hour;
 
                 # algorithm from Claus Tøndering
                 my int $a = (14 - nqp::atpos_i($lt,4)) div 12;
@@ -33,7 +34,7 @@ my class DateTime does Dateish {
                 $TZ-offset = (
                   ($jd - 2440588) * 86400
                     + nqp::atpos_i($lt,2) * 3600
-                    + $last-minute-seen * 60
+                    + nqp::atpos_i($lt,1) * 60
                     + nqp::atpos_i($lt,0)
                 ) - $utc
             }