Correcting dates on image files

#!/usr/bin/env ruby
 
require 'time'
 
# 2009-10-17 LHN: Adjust file modification time and EXIF fields of files.
# Used to correct files that were recorded on a camera that had it's clock
# accidentally reset.
 
# Requires ExifTool (http://www.sno.phy.queensu.ca/~phil/exiftool)
 
def help_message
  puts <<HEREDOC
 
Camera Date rev 2009-10-18
 
Camera Date is intended to correct images take on cameras whose clocks were
accidentally reset. It will update the following pieces of information:
  * filesystem timestamp
  * EXIF fields DateTimeOriginal, CreateDate, ModifyDate, Date
 
Syntax is camera_date.rb bad_date good_date file ...
 
bad_date is the date of an image as recorded by the camera
good_date is the correct date for that image
 
bad_date and good_date are used to calculate an ofset that can then be applied
to all specified files.
 
dates are specified in RFC8288 format and must be surrounded by quotes.
 
Example:
camera_date.rb "18 Jan 2005 22:10:03" "14 Sep 2009 10:10:03" IMGP7843.PEF IMGP7845.PEF
 
HEREDOC
  exit
end
 
help_message unless ARGV.length >= 3
 
begin
  INPUT_DATE = Time.parse(ARGV[0])
  OUTPUT_DATE = Time.parse(ARGV[1])
  OFFSET_TOTAL_SECS = OUTPUT_DATE.to_i - INPUT_DATE.to_i
  puts "#{INPUT_DATE} => #{OUTPUT_DATE}"
  puts "Offset in seconds is #{OFFSET_TOTAL_SECS}" 
 
  ARGV[2 .. -1].each{ |entry| 
    mtime = File.stat(entry).mtime
    newtime = mtime + OFFSET_TOTAL_SECS
    puts "#{entry}: #{mtime} -> #{newtime}"
    system("exiftool \"-AllDates=#{newtime.strftime("%Y:%m:%d %H:%M:%S")}\" \"-Date=#{newtime.strftime("%Y:%m:%d")}\" #{entry}")
    system("touch -c -t#{newtime.strftime("%Y%m%d%H%M.%S")} #{entry}")
  }
  puts "Done."
rescue StandardError => e
  puts e
  help_message
end