JSON API: fix date format issue

we cannot use the getTimestamp() method because it is not supported in
PHP < 5.3.  Instead we'll use strtotime()
JSON API: fix date format issue

Instead of strtotime, we'll use date_format
This may need a rebase before we merge.
Merge pull request #1333 from Automattic/fix/issue-1326

JSON API: fix date format issue