Does anyone know why?

datetime-omg.rb

require 'date'

# 1) this doesn't work, why?
format = "%-m/%-d/%Y %-l:%M:%S %p"
d = DateTime.new(2014,6,5,1,5,9)
d == DateTime.strptime(d.strftime(format), format) # raised ArgumentError: invalid date

# 2) this works, why?
format = "%-m/%-d/%Y %-l:%M:%S %p"
format2 = "%m/%d/%Y %l:%M:%S %p"
d = DateTime.new(2014,6,5,1,5,9)
d == DateTime.strptime(d.strftime(format), format2)

I'm not actually sure why do you expect it to work.

irb(main):058:0> DateTime.new(2014,6,5,1,5,9).strftime("%-m/%-d/%Y %-l:%M:%S %p")
=> "6/5/2014 1:05:09 AM"
irb(main):059:0> DateTime.new(2014,6,5,1,5,9).strftime("%m/%d/%Y %l:%M:%S %p")
=> "06/05/2014  1:05:09 AM"

Using these 2 different formats, produces different results. When you call strptime on them, you need to provide exact format of the string. So DateTime.strptime(d.strftime(format), format) will work, DateTime.strptime(d.strftime(format2), format2) will work, but DateTime.strptime(d.strftime(format), format2) won't because the result of strftime does not match format of strptime 2nd argument.

@arnvald, hmm, I tried running 1) and 2) on Ruby 2.1:

2. DateTime.strptime(d.strftime(format), format2) actually works.

3. DateTime.strptime(d.strftime(format), format) does not work.

Do you get the same results?

@arnvald, I found this:
https://www.ruby-forum.com/topic/4416958

http://www.ruby-doc.org/stdlib-2.1.2/libdoc/date/rdoc/DateTime.html#method-c-strptime
"strptime does not support specification of flags and width unlike strftime."

(:mindblown:)