gistfile1.rb

require "rubygems"
require "timecode"
$KCODE = 'u'
SRT_TIMECODE_FORMAT = /((\d{2}):(\d{2}):(\d{2}),(\d+))/

class SRT_Timecode < Timecode
  def to_s
    with_fractional_seconds.gsub(/\.(\d+)/, ',\1')
  end
  
  def self.parse_with_fractional_seconds(some_tc, fps = Timecode::DEFAULT_FPS)
    super(some_tc.gsub(/,/, '.'), fps)
  end
end

# Get the slip amount
slip = -25

subtitle_file = File.read("/path/to/srt")

subtitle_file.gsub!(SRT_TIMECODE_FORMAT) do | a_tc |
  SRT_Timecode.parse_with_fractional_seconds(a_tc) + slip
end

File.open("/path/to/srt_out", "w") do | f |
  f << subtitle_file
end