RESULT=7&RPREF=R1357D486E3F&RESPMSG=Field format error: Invalid START/NEXTPAYMENTDATE field

The exact order of the submitted variables is important:

        'TRXTYPE' => 'R',
        'PARTNER' => $API_Partner,
        'VENDOR' => $API_Vendor,
        'USER' => $API_User,
        'PWD' => $API_Password,
        'ACTION' => $action

And then any others required depending on which action. For the Create action, START must be tomorrow's date or a date in the future. Hope this helps somebody else having problems with PayPal's PayFlow API.

use

START='12152012'

instead of START=12152012