Using API to retrieve contact records by modified date
You can use modified_date in Contact.get eg:

Return the date:

$result = civicrm_api3('Contact', 'get', array(
  'sequential' =&gt; 1,
  'return' =&gt; array("modified_date"),
));


Search for Contacts modified after a date:

$result = civicrm_api3('Contact', 'get', array(
  'sequential' =&gt; 1,
  'modified_date' =&gt; array('&gt;' =&gt; "2017-04-12"),
));


The API explorer is useful for exploring things like this.