Date format issue: Empty (NULL) date is getting save in mysql DB

I am having issue while saving date (effective_at) to mysql databse
usning format MM-DD-YY. The date is getting saved as null but while
using format 'YYYY-MM-DD' its working perfectly. Please see below.

Processing UsersController#update (for 127.0.0.1 at 2009-07-20
10:35:24) [PUT]
  Session ID:
BAh7ByIKZmxhc2hJQzonQWN0aW9uQ29udHJvbGxlcjo6Rmxhc2g6OkZsYXNo
%0ASGFzaHsABjoKQHVzZWR7ADoMdXNlcl9pZGkI--454ad118c53a4bb341e1cf4af1fabbf5eee6cc4b
  Parameters: {"user"=>{"effective_at"=>"07-19-2009", "status"=>"I",
"expired_at"=>"", "password_confirmation"=>"", "reset_password"=>"N",
"id"=>"1", "first_name"=>"abcsss", "group"=>"S",
"last_name"=>"testwwwww", "password"=>"", "login"=>"quentin",
"middle_name"=>"wwww", "email"=>"quentin@gmail.com", "agent_id"=>"001-
AMC-123456"}, "format"=>"xml", "_method"=>"PUT", "action"=>"update",
"id"=>"1", "controller"=>"users"}
  e[4;35;1mUser Columns (0.000000)e[0m e[0mSHOW FIELDS FROM `users`e
[0m
  e[4;36;1mUser Load (0.015000)e[0m e[0;1mSELECT * FROM `users`
WHERE (`users`.`id` = 3) LIMIT 1e[0m
  e[4;35;1mUser Load (0.000000)e[0m e[0mSELECT * FROM `users` WHERE
(`users`.`id` = 1) e[0m
  e[4;36;1mSQL (0.000000)e[0m e[0;1mBEGINe[0m
  e[4;35;1mUser Load (0.000000)e[0m e[0mSELECT * FROM `users` WHERE
(LOWER(users.login) = 'quentin' AND users.id <> 1) LIMIT 1e[0m
  e[4;36;1mUser Load (0.000000)e[0m e[0;1mSELECT * FROM `users`
WHERE (LOWER(users.email) = 'quentin@gmail.com' AND users.id <> 1)
LIMIT 1e[0m
  e[4;35;1mUser Update (0.000000)e[0m e[0mUPDATE `users` SET
`created_at` = '2009-01-13 12:17:58', `agent_id` = '001-AMC-123456',
`login` = 'quentin', `expired_at` = NULL, `reset_password` = 'N',
`remember_token_expires_at` = NULL, `status` = 'I', `salt` =
'7e3041ebc2fc05a40c60028e2c4901a81035d3cd', `effective_at` =
'2009-07-21 00:00:00', `middle_name` = 'wwww', `last_name` =
'testwwwww', `group` = 'S', `first_name` = 'abcsss', `remember_token`
= NULL, `email` = 'quentin@gmail.com', `updated_at` = '2009-07-20
10:35:24', `encrypted_password` =
'd084e486e59a0f8af8cc6a2d730097ba0404b008' WHERE `id` = 1e[0m
  e[4;36;1mSQL (0.000000)e[0m e[0;1mCOMMITe[0m
WARNING: Can't mass-assign these protected attributes: id
  e[4;35;1mSQL (0.000000)e[0m e[0mBEGINe[0m
  e[4;36;1mUser Load (0.000000)e[0m e[0;1mSELECT * FROM `users`
WHERE (LOWER(users.login) = 'quentin' AND users.id <> 1) LIMIT 1e[0m
  e[4;35;1mUser Load (0.000000)e[0m e[0mSELECT * FROM `users` WHERE
(LOWER(users.email) = 'quentin@gmail.com' AND users.id <> 1) LIMIT 1e
[0m
  e[4;36;1mUser Update (0.000000)e[0m e[0;1mUPDATE `users` SET
`created_at` = '2009-01-13 12:17:58', `agent_id` = '001-AMC-123456',
`login` = 'quentin', `expired_at` = NULL, `reset_password` = 'N',
`remember_token_expires_at` = NULL, `status` = 'I', `salt` =
'7e3041ebc2fc05a40c60028e2c4901a81035d3cd', `effective_at` = NULL,
`middle_name` = 'wwww', `last_name` = 'testwwwww', `group` = 'S',
`first_name` = 'abcsss', `remember_token` = NULL, `email` =
'quentin@gmail.com', `updated_at` = '2009-07-20 10:35:24',
`encrypted_password` = 'd084e486e59a0f8af8cc6a2d730097ba0404b008'
WHERE `id` = 1e[0m
  e[4;35;1mSQL (0.000000)e[0m e[0mCOMMITe[0m
Completed in 0.10900 (9 reqs/sec) | Rendering: 0.00000 (0%) | DB:
0.01500 (13%) | 200 OK [http://localhost/users/1.xml]