Script parses and prints date time as several formats

d.rb

#!/usr/bin/env ruby
#
# d - Parse and print date time as several formats
#
# Usage:
#
# $ d [DATETIME]
#
# Supported formats:
#
# - Seconds or milliseconds from 1970-01-01T00:00:00Z as decimal integer.
# - ISO8601 / RFC2822 / JIS X 0301 date time format.
# - If argument is ommited, use current time.
#
# Output formats:
#
# - ISO8601 (local)
# - ISO8601 (UTC)
# - RFC2822 (local)
# - RFC2822 (UTC)
# - JIS X 0301 (local)
# - Seconds from 1970-01-01T00:00:00Z
# - Milliseconds from 1970-01-01T00:00:00Z
#
# Example:
#
# $ d 1450159583
# 2015-12-15T15:06:23+09:00
# 2015-12-15T06:06:23Z
# Tue, 15 Dec 2015 15:06:23 +0900
# Tue, 15 Dec 2015 06:06:23 -0000
# H27.12.15T15:06:23+09:00
# 1450159583
# 1450159583123
#
# $ d 2015-12-15T15:06:23+09:00
# (same as above)

require "time"

SECONDS_AT_2000 = 946684800 # Seconds from 1970-01-01T00:00:00Z to 2000-01-01T00:00:00Z

def parse(s)
  s = s.to_s.dup
  s.strip!

  if s =~ /\A\d+\z/ # Integer
    i = s.to_i
    case
    when i > SECONDS_AT_2000 * 1_000
      return Time.at(i.to_f / 1_000)
    when i > SECONDS_AT_2000
      return Time.at(i)
    end
  end

  if s =~ /\A\d+\.\d+\z/
    f = s.to_f
    return Time.at(f)
  end

  begin
    return Time.parse(s)
  rescue ArgumentError
  end

  raise "Cannot parse #{s.inspect}"
end

def print(t)
  puts t.getlocal.iso8601
  puts t.utc.iso8601
  puts t.getlocal.rfc2822
  puts t.utc.rfc2822
  puts t.getlocal.to_datetime.jisx0301
  puts t.to_i
  puts (t.to_f * 1_000).to_i
end

time = parse(ARGV.first || Time.now.to_f)
print(time)