Date formatting in WS Form follows the same conventions used when formatting PHP dates. For a list of available formatting string options, see the PHP date manual page.
Variables that use date formatting strings are as follows:
#post_date_custom("formatting string")
#server_date_custom("formatting string")
#blog_date_custom("formatting string")
#client_date_custom("formatting string")
For example:
#server_date_custom("m/d/Y H:i:s")