This would be useful.
But, correctly identifying document dates is difficult to automate. So, even if the functionality were there. You might not get the results you want.
API users can refer to the date field they find most practical.
// Date last modified timestamp.
"timestamp":1184044740,
// Date last modified formatted.
"date":"Tue Jul 10 05:19:00 2007",
// Date published timestamp.
"pdate":1184044740,
// Date crawled timestamp.
"cdatetimestamp":1184044740,
// Date crawled formatted.
"cdate":"Tue Jul 10 05:19:00 2007",