From: Peter Chines <pchines@nhgri.nih.gov>
Date: Wed, 27 Jan 1999 16:11:31 +0000 (-0500)
Subject: several bug fixes; now croak()s when date exceeds integer limits (instead
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=06ef4121413231b19bf176ccf514d79951c10a41;p=p5sagit%2Fp5-mst-13.2.git

several bug fixes; now croak()s when date exceeds integer limits (instead
of silently returning bogus values)
	Message-Id: <199901272117.QAA21458@kronos.nhgri.nih.gov>
	Subject: Time::Local

p4raw-id: //depot/perl@2918
---

diff --git a/lib/Time/Local.pm b/lib/Time/Local.pm
index b2fba7c..2534028 100644
--- a/lib/Time/Local.pm
+++ b/lib/Time/Local.pm
@@ -6,57 +6,33 @@ use Carp;
 @ISA = qw(Exporter);
 @EXPORT = qw(timegm timelocal);
 
-=head1 NAME
-
-Time::Local - efficiently compute time from local and GMT time
-
-=head1 SYNOPSIS
-
-    $time = timelocal($sec,$min,$hours,$mday,$mon,$year);
-    $time = timegm($sec,$min,$hours,$mday,$mon,$year);
-
-=head1 DESCRIPTION
-
-These routines are quite efficient and yet are always guaranteed to
-agree with localtime() and gmtime(), the most notable points being
-that year is year-1900 and month is 0..11.  We manage this by caching
-the start times of any months we've seen before.  If we know the start
-time of the month, we can always calculate any time within the month.
-The start times themselves are guessed by successive approximation
-starting at the current time, since most dates seen in practice are
-close to the current date.  Unlike algorithms that do a binary search
-(calling gmtime once for each bit of the time value, resulting in 32
-calls), this algorithm calls it at most 6 times, and usually only once
-or twice.  If you hit the month cache, of course, it doesn't call it
-at all.
-
-timelocal is implemented using the same cache.  We just assume that we're
-translating a GMT time, and then fudge it when we're done for the timezone
-and daylight savings arguments.  The timezone is determined by examining
-the result of localtime(0) when the package is initialized.  The daylight
-savings offset is currently assumed to be one hour.
-
-Both routines return -1 if the integer limit is hit. I.e. for dates
-after the 1st of January, 2038 on most machines.
-
-=cut
-
-BEGIN {
+# Set up constants
     $SEC  = 1;
     $MIN  = 60 * $SEC;
     $HR   = 60 * $MIN;
     $DAY  = 24 * $HR;
-    $epoch = (localtime(2*$DAY))[5];	# Allow for bugs near localtime == 0.
-
-    $YearFix = ((gmtime(946684800))[5] == 100) ? 100 : 0;
-
-}
+# Determine breakpoint for rolling century
+    my $thisYear = (localtime())[5];
+    $nextCentury = int($thisYear / 100) * 100;
+    $breakpoint = ($thisYear + 50) % 100;
+    $nextCentury += 100 if $breakpoint < 50;
 
 sub timegm {
-    $ym = pack(C2, @_[5,4]);
-    $cheat = $cheat{$ym} || &cheat;
-    return -1 if $cheat<0 and $^O ne 'VMS';
-    $cheat + $_[0] * $SEC + $_[1] * $MIN + $_[2] * $HR + ($_[3]-1) * $DAY;
+    my (@date) = @_;
+    if ($date[5] > 999) {
+        $date[5] -= 1900;
+    }
+    elsif ($date[5] >= 0 && $date[5] < 100) {
+        $date[5] -= 100 if $date[5] > $breakpoint;
+        $date[5] += $nextCentury;
+    }
+    $ym = pack(C2, @date[5,4]);
+    $cheat = $cheat{$ym} || &cheat(@date);
+    $cheat
+    + $date[0] * $SEC
+    + $date[1] * $MIN
+    + $date[2] * $HR
+    + ($date[3]-1) * $DAY;
 }
 
 sub timelocal {
@@ -66,11 +42,11 @@ sub timelocal {
     my (@lt) = localtime($t);
     my (@gt) = gmtime($t);
     if ($t < $DAY and ($lt[5] >= 70 or $gt[5] >= 70 )) {
-      # Wrap error, too early a date
-      # Try a safer date
-      $tt = $DAY;
-      @lt = localtime($tt);
-      @gt = gmtime($tt);
+	# Wrap error, too early a date
+	# Try a safer date
+	$tt = $DAY;
+	@lt = localtime($tt);
+	@gt = gmtime($tt);
     }
 
     my $tzsec = ($gt[1] - $lt[1]) * $MIN + ($gt[2] - $lt[2]) * $HR;
@@ -89,7 +65,6 @@ sub timelocal {
     $tzsec += $HR if($lt[8]);
     
     $time = $t + $tzsec;
-    return -1 if $cheat<0 and $^O ne 'VMS';
     @test = localtime($time + ($tt - $t));
     $time -= $HR if $test[2] != $_[2];
     $time;
@@ -97,8 +72,6 @@ sub timelocal {
 
 sub cheat {
     $year = $_[5];
-    $year -= 1900
-    	if $year > 1900;
     $month = $_[4];
     croak "Month '$month' out of range 0..11"	if $month > 11 || $month < 0;
     croak "Day '$_[3]' out of range 1..31"	if $_[3] > 31 || $_[3] < 1;
@@ -107,7 +80,6 @@ sub cheat {
     croak "Second '$_[0]' out of range 0..59"	if $_[0] > 59 || $_[0] < 0;
     $guess = $^T;
     @g = gmtime($guess);
-    $year += $YearFix if $year < $epoch;
     $lastguess = "";
     $counter = 0;
     while ($diff = $year - $g[5]) {
@@ -115,7 +87,8 @@ sub cheat {
 	$guess += $diff * (363 * $DAY);
 	@g = gmtime($guess);
 	if (($thisguess = "@g") eq $lastguess){
-	    return -1; #date beyond this machine's integer limit
+	    croak "Can't handle date (".join(", ",@_).")";
+	    #date beyond this machine's integer limit
 	}
 	$lastguess = $thisguess;
     }
@@ -124,13 +97,15 @@ sub cheat {
 	$guess += $diff * (27 * $DAY);
 	@g = gmtime($guess);
 	if (($thisguess = "@g") eq $lastguess){
-	    return -1; #date beyond this machine's integer limit
+	    croak "Can't handle date (".join(", ",@_).")";
+	    #date beyond this machine's integer limit
 	}
 	$lastguess = $thisguess;
     }
     @gfake = gmtime($guess-1); #still being sceptic
     if ("@gfake" eq $lastguess){
-	return -1; #date beyond this machine's integer limit
+        croak "Can't handle date (".join(", ",@_).")";
+        #date beyond this machine's integer limit
     }
     $g[3]--;
     $guess -= $g[0] * $SEC + $g[1] * $MIN + $g[2] * $HR + $g[3] * $DAY;
@@ -138,3 +113,98 @@ sub cheat {
 }
 
 1;
+
+__END__
+
+=head1 NAME
+
+Time::Local - efficiently compute time from local and GMT time
+
+=head1 SYNOPSIS
+
+    $time = timelocal($sec,$min,$hours,$mday,$mon,$year);
+    $time = timegm($sec,$min,$hours,$mday,$mon,$year);
+
+=head1 DESCRIPTION
+
+These routines are the inverse of built-in perl fuctions localtime()
+and gmtime().  They accept a date as a six-element array, and return
+the corresponding time(2) value in seconds since the Epoch (Midnight,
+January 1, 1970).  This value can be positive or negative.
+
+It is worth drawing particular attention to the expected ranges for
+the values provided.  While the day of the month is expected to be in
+the range 1..31, the month should be in the range 0..11.  
+This is consistent with the values returned from localtime() and gmtime().
+
+Strictly speaking, the year should also be specified in a form consistent
+with localtime(), i.e. the offset from 1900.
+In order to make the interpretation of the year easier for humans,
+however, who are more accustomed to seeing years as two-digit or four-digit
+values, the following conventions are followed:
+
+=over 4
+
+=item *
+
+Years greater than 999 are interpreted as being the actual year,
+rather than the offset from 1900.  Thus, 1963 would indicate the year
+of Martin Luther King's assassination, not the year 2863.
+
+=item *
+
+Years in the range 100..999 are interpreted as offset from 1900, 
+so that 112 indicates 2012.  This rule also applies to years less than zero
+(but see note below regarding date range).
+
+=item *
+
+Years in the range 0..99 are interpreted as shorthand for years in the
+rolling "current century," defined as 50 years on either side of the current
+year.  Thus, today, in 1999, 0 would refer to 2000, and 45 to 2045,
+but 55 would refer to 1955.  Twenty years from now, 55 would instead refer
+to 2055.  This is messy, but matches the way people currently think about
+two digit dates.  Whenever possible, use an absolute four digit year instead.
+
+=back
+
+The scheme above allows interpretation of a wide range of dates, particularly
+if 4-digit years are used.  
+Please note, however, that the range of dates that can be actually be handled
+depends on the size of an integer (time_t) on a given platform.  
+Currently, this is 32 bits for most systems, yielding an approximate range 
+from Dec 1901 to Jan 2038.
+
+Both timelocal() and timegm() croak if given dates outside the supported
+range.
+
+=head1 IMPLEMENTATION
+
+These routines are quite efficient and yet are always guaranteed to agree
+with localtime() and gmtime().  We manage this by caching the start times
+of any months we've seen before.  If we know the start time of the month,
+we can always calculate any time within the month.  The start times
+themselves are guessed by successive approximation starting at the
+current time, since most dates seen in practice are close to the
+current date.  Unlike algorithms that do a binary search (calling gmtime
+once for each bit of the time value, resulting in 32 calls), this algorithm
+calls it at most 6 times, and usually only once or twice.  If you hit
+the month cache, of course, it doesn't call it at all.
+
+timelocal() is implemented using the same cache.  We just assume that we're
+translating a GMT time, and then fudge it when we're done for the timezone
+and daylight savings arguments.  Note that the timezone is evaluated for
+each date because countries occasionally change their official timezones.
+Assuming that localtime() corrects for these changes, this routine will
+also be correct.  The daylight savings offset is currently assumed 
+to be one hour.
+
+=head1 BUGS
+
+The whole scheme for interpreting two-digit years can be considered a bug.
+
+Note that the cache currently handles only years from 1900 through 2155.
+
+The proclivity to croak() is probably a bug.
+
+=cut
diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index 24924cd..f64a1da 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -229,6 +229,12 @@ The accessors methods Re, Im, arg, abs, rho, theta, methods can
 A little bit of radial trigonometry (cylindrical and spherical) added,
 for example the great circle distance.
 
+=item Time::Local
+
+The timelocal() and timegm() functions used to silently return bogus
+results when the date exceeded the machine's integer range.  They
+consistently croak() if the date falls in an unsupported range.
+
 =back
 
 =head2 Pragmata