Demonstrating an issue round-tripping between Syck and Psych. Clone then run syck-psych-syck.sh.

duration_updater.rb

# I'm not sure why but sometimes Syck is used for 'yaml' even under 1.9.2. This forces the issue.
Y = 
  if RUBY_VERSION =~ /1.9/
    require 'psych'
    Psych
  else
    require 'yaml'
    YAML
  end

FILENAME = 'durations.yml'

$stderr.puts "Updating #{FILENAME} under\n  #{RUBY_DESCRIPTION}"

d =
  if File.exist?(FILENAME)
    Y.load(File.read(FILENAME))
  else
    {}
  end

d[d.size + 1] = ARGV[0]

Y.dump(d, File.open(FILENAME, 'w'))

output.txt

$ ./syck-psych-syck.sh 
Updating durations.yml under
  ruby 1.8.7 (2011-06-30 patchlevel 352) [i686-darwin10.8.0]
--- 
1: "01:03:05"
Updating durations.yml under
  ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin10.8.0]
---
1: 01:03:05
2: 02:04:03
Updating durations.yml under
  ruby 1.8.7 (2011-06-30 patchlevel 352) [i686-darwin10.8.0]
--- 
1: 3785
2: 7443
3: "03:05:10"

syck-psych-syck.sh

#!/bin/bash -e

FILENAME=durations.yml

rm -f $FILENAME
rvm 1.8.7 ruby duration_updater.rb '01:03:05'
cat $FILENAME
rvm 1.9.2 ruby duration_updater.rb '02:04:03'
cat $FILENAME
rvm 1.8.7 ruby duration_updater.rb '03:05:10'
cat $FILENAME