After much trial and error and checking suite answers for numerous responses, i found this one that helped get me in the right direction (solution 2)
https://suiteanswers.custhelp.com/app/answers/detail/a_id/87829/loc/en_US
nlapiDateToString(nlapiAddDays(nlapiStringToDate(nlapiGetFieldValue('trandate')), parseInt(nlapiGetLineItemValue('item', 'custcol_default_lead_time', line))), 'date') i feel like am maybe close but stuck at how to proceed.
I also tried nlapiDateToString(nlapiAddDays(nlapiStringToDate(nlapiGetFieldValue('trandate')), parseInt(nlapiGetLineItemValue('item', 'custcol_default_lead_time', line))), 'DD/MM/YYYY')
The error i am getting is "Invalid date value (must be DD/MM/YYYY)"
I did find another suite answer to indicate that DD/MM/YYYY format in the preferences may not be compatible with this i am really hoping there is another way as we need to avid getting a script for this. surely it it is possible with a workflow?) Customer account preferences are DD/MM/YYYY (we are in Australia so this is the correct format for this country)
https://suiteanswers.custhelp.com/app/answers/detail/a_id/23304/loc/en_US
If i do not use a custom field for the lead time and instead use a hard coded number (ie 30) then the above works. So it only mucks up when i add the custom field.
The last part of this is i actually need to source "Today"s date when doing this, not trandate, but i wanted to solve lead time part first, but if anyone can help with both that would be awesome.