Using SuiteScript to enter static ship date

I have created a transaction body field for original ship date so our production team knows what the actual ship date was since it does get pushed back on some sales orders. My first script ended up copying the date from the NS 'ship date' field, but updates when that field gets updated by sales.

I tried putting an "if" statement in the script on the custom field to check if it's null and then copy the original ship date into the field upon sales order creation.

Here's what I have thus far, comments are welcome:

function SetOSDate (field) {
    var sdate = nlapiFieldValue('shipdate');