Showing date parsing problem using "%m/%d/%Y" as format. According to system level documentation (man 3 strptime) the formats between strftime and strptime should be reflexive. At the clib level, they are, but at the ruby level, they are not.

ruby-2.0.0-example

$ irb -r date
irb(main):001:0> format = "%m/%d/%Y"
=> "%m/%d/%Y"
irb(main):002:0> Date.today.strftime(format)
=> "08/18/2013"
irb(main):003:0> Date.parse(Date.today.strftime(format),format)
ArgumentError: invalid date
	from (irb):3:in `parse'
	from (irb):3
	from /Users/tamara/.rubies/ruby-2.0.0-p427/bin/irb:12:in `<main>'
irb(main):004:0> Date.today.to_s
=> "2013-08-18"
irb(main):005:0> Date.parse(Date.today.to_s)
=> #<Date: 2013-08-18 ((2456523j,0s,0n),+0s,2299161j)>
irb(main):006:0> quit
$ ruby -v
ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.4.0]
$ irb -v
irb 0.9.6(09/06/30)

test output

$ ./test
Current time from strftime: 08/18/2013
Parsed time: 
Month:	   8
Day:	  18
Year:	2013

test.c

#include <stdio.h>
#include <time.h>

int main(void)
{
  time_t now;
  now = time(NULL);

  struct tm *now_tm;
  now_tm = gmtime(&now);

  char *format = "%m/%d/%Y";
  char buf[25];
  strftime(buf, sizeof(buf), format, now_tm);

  printf("Current time from strftime: %s\n", buf);

  struct tm now2_tm;
  strptime(buf, format, &now2_tm);
  
  printf("Parsed time: \n");
  printf("Month:\t%4d\n",now2_tm.tm_mon+1);
  printf("Day:\t%4d\n",now2_tm.tm_mday);
  printf("Year:\t%4d\n",now2_tm.tm_year+1900);
}