How to tell if a user has updated a profile field in Drupal 7

I am trying to determine how in drupal 7 I can programmatically tell if a user has updated a custom field in their profile within the past 24 hours however looking through the tables I can't seem to find any field with that information does anyone have any ideas?? Essentially I need to pull this information out during a batch job that run开发者_如何学运维 onces a night and updates other systems based upon the information in drupal. If someone can think of a better way todo this it would be appreciated. thanks for any help you can provide

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ok i dont know if its working or not try the following steps

1- first you should tell drupal....when user edit the profile page do something...(whatever the something is ..insert something to database..retrieve something..etc)

/**
* Implementation of hook_form_alter
*/
function newcontent_form_alter(&$form, &$form_state, $form_id){
if($form_id == 'user_profile_form'){
    $form['#submit'][]  = '_do_something_when_user_save_profile';   
}    
}   


/**
* do something when user save profile
*/
function _do_something_when_user_save_profile($form,$form_state){
// do something
}   

now i think we need to do some queries ... first we need to create 2 tables http://api.drupal.org/api/drupal/modules--system--system.api.php/function/hook_schema/7 (this url will help you to create your schema)

• the first table will contain the current value of the field u want track so i think this table should have the following fields (primary key , user id, field name, field value)

• the second table will contain the date of the last upated operation the user made i think the fields will be like the following (primary key,user id , field name, date)

now lets return to the form submit function

/**
* do something when user save profile
*/
function _do_something_when_user_save_profile($form,$form_state){
   // now i can check in the first table to see
       // is the current submitted value for this field = the value in the table
       // if its = the table value then the user didn't change any thing now i dont 
       // need to update the second table
       // if the current submitted value != the value in the table then its mean
       // that the user have updated this field ... 
       // now i should insert new value to the second 
       // table with the current date time and the current field value
}   

i hope u under stand me and sorry for my bad english

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maged adel provided reasonable solution, however there is one more way of doing this: you can use hook_user_update for this purpose. It's better that solution provided above because of 2 reasons: 1) you have both $edit - entered values and $account - user account values, so you can compare and get know what fields being updated (if you don't have 1 specific field to check). 2) just 1 hook to implement.