Converting to a numeric value and adding dates

I'm trying to add a number to a date using the adddate function in Zoho CRM. The number is to be converted from a string function value. Have used the following formulae, but it just doesn't work:

Formula 1

Adddate(${PurchaseOrders.PO Date},${PurchaseOrders.Pure Production Timelines}.toLong(),'DAY')

Formula 2

Adddate(${PurchaseOrders.PO Date},PurchaseOrders.Pure Production Timelines.toLong(),'DAY')

Formula 3

numeric=${PurchaseOrders.Pure Production Timelines}.toLong();

Adddate(${PurchaseOrders.PO Date},numeric,'DAY')

Formula 4

numeric=PurchaseOrders.Pure Production Timelines.toLong();

Adddate(${PurchaseOrders.PO Date},numeric,'DAY')

Please advise.