Computing small time differences in Singapore adds 30 minutes when converted to hh:mm:ss.mmm

Question

I have this Perl script where I need to monitor the execution time of DBI calls.

In Europe (France), I have no problem: 2 seconds execution time is reported 2 seconds.

This same script running on a computer in Singapore reports 30 minutes and 2 seconds.

Why ?

use strict;
use Time::Format qw(%time);
use Time::HiRes qw(gettimeofday);

my $time_start = gettimeofday();

sleep 2;  # some action goes here

my $stat_perf = gettimeofday() - $time_start;
print STDOUT $time{'mm:ss.mmm', $stat_perf} . " \n";

The output in France is

00:02.000 

The same script running in Singapore yields:

30:02.001 

Why ?

Answer 1

The anser is ...

Singapore is now 08h00 offset from UTC. In 1970, it was offset by 08h30. Asking for the conversion of a few seconds into a string will get us to 1970, not today's date, and timezone.

By requesting

print STDOUT $time{'mm:ss.mmm', 2} . " \n";

the system adjusts to 1970 (epoch) timezone offset.

In order to get a correct result in Singapore, we must shift to after 1982, when Singapore made its last timezone change.

print STDOUT $time{'mm:ss.mmm', 2 + 1356994800} . " \n";

as

UNIX_TIMESTAMP('2013-01-01 00:00:00')  = 1356994800

We are only concerned by the time of day portion of the date, so this does it.

Check with

zdump -v Asia/Singapore

This is the trick.

Answer 2

According to this documentation, the gettimeofday function returns seconds or microseconds since the unix epoch, which is 1/1/1970 UTC. Because it is in UTC, it is not affected by time zones at all.

Also, in your original code you are just using gettimeofday, which is going to be returning timestamps from now, not from 1970. But in your suggested answer, for some reason, you have hard-set the timestamp, which won't help you do much.

Yes, there is history to just about every time zone, including Singapore. You can see it in the TZDB here. But you are incorrect about it being +8:30 at the epoch. It was actually +7:30. You can verify also on this site. But it doesn't matter anyway because like I said, gettimeofday works strictly in UTC.

I think the problem is in how you are interpreting the results. You have as your last line:

print STDOUT $time{'mm:ss.mmm', $stat_perf} . " \n";

But $stat_perf is the elapsed duration of time, not a value that you can treat as a timestamp. You probably shouldn't be passing it to $time, since that will use the local time zone and be expecting a full timestamp.

Also, you may want to use tv_interval, as shown in the examples.

Update

I searched through the CPAN archives and I'm sure somewhere there is a module for formatting an elapsed duration of time, but I can't seem to find it. Anyway, it's not too hard to write this on your own. Here, this should work:

my $min = $stat_perf / 60;
my $sec = ($stat_perf * 1000 % 60000) / 1000;
my $elapsed = sprintf("%02u:%06.3f", $min, $sec);
print STDOUT $elapsed . "\n";

Answer 3

Here is a script that emulates $time{} in converting a real number into a string representing the mm:ss sexagesimal conversion of its integer part, concatenated with the decimal remainder formatted as microseconds.

As this is going to be part of a library, there are protections set to avoid invoking it with bad arguments.

I hope I didn't miss something.

use strict;
use Time::Format qw(%time);


# ----------------------------------------------------------
# A substitute to $time{} as we have issues with TZ offsets at epoch days in some part of the World
# A real to sexagesimal converter
# Format will be set to match $time{'mm:ss.mmm', $stat_perf};
sub microTime {
  return '' unless (my ($intertime) = @_);
  return '' unless (ref ($intertime) eq '');
  return '' unless (sprintf("%s", $intertime) =~ m/^(?:[\d]+)(?:\.(?:[\d]+))?$/);
  my $intNum = int($intertime);
  "a" =~ /a/; # Resets regex buffers
  sprintf ("%.03f", $intertime - $intNum) =~ m,\.([\d]+),;
  my $intDec = $1;  # It's always defined
  my $intUnder = $intNum % 3600;
  my $intMin = int($intUnder / 60);
  my $intSec = $intUnder % 60;
  return sprintf ("%02d:%02d.%03d", $intMin, $intSec, $intDec);
}


  my $stat_perf;

  $stat_perf = 345.987;
  $stat_perf = 345;
  $stat_perf = 3945.987;
  $stat_perf = 0;
  $stat_perf = 3945.918733;


  print STDOUT sprintf (" >> %s\n", &microTime ($stat_perf));
  print STDOUT sprintf (" == %s\n", $time{'mm:ss.mmm', $stat_perf});