Calculating Next Invoice Requisition Date

i Am Invocing our Clients every 2 weeks "Biweekly "

I have A sheet set up like this

Managers Name who Reprots progress Done

Next i have a Coulmn called Date send
Which is the Date we emailed The Invoice /Req To our customer 

Then i have Next Req Date Which  should be calculated 2 weeks after the prior Send date 

Here is my issue how can i calulate Automaticaly when we need to Create neext req /Invoice 

which Typicaly will be 2 weeks after send Date