need the number of seconds since....

AWS recommends storing dates in ISO 8601:
https://docs.aws.amazon.com/amazondynamo....DataTypes
http://en.wikipedia.org/wiki/ISO_8601

2016-02-15
2015-12-21T17:42:34Z
20150311T122706Z

http://php.net/manual/en/class.datetime.php
http://php.net/manual/en/datetime.createfromformat.php

PHP Code:

echo DateTime::createFromFormat(DateTime::ATOM,'2015-12-21T17:42:34Z')->format('U');