Here's a work around that I implemented in PAM that you maybe able to move into Catalog using the script box on the form. It accepts a date passed in human readable format and converts it to unix time to be passed into SD. I don't know which format your other system uses so this may require some tweaking.
To use this in the form you'd need to use the ca_fdGetDateFieldValue function (in the onChange attribute) to retrieve the fields values and pass them into your function in the script box.
//converts Unix timestamps passed in from catalog to the proper time zone.
function convertToEpoch(dateVal){
var date = new Date(dateVal).getTime(); //convert the human readable format to Unix time
var toEpoch = date.toString().substring(0, 10); //convert the date to string to get a substring of it(eliminating the JS milliseconds)
return toEpoch;
}