Modify email order confirmation with dynamic date/time


I want to modify the order confirmation emails that get sent to customers when they make a purchase. The email template can be found in Product->Store Settings ->Messages.

I am running a pick up/collect service, so I want to modify the email so that I can tell customers what date to pick up their orders.

To do this i'd need to calculate the date/time of when the order is placed, then determine from that when the next pick up day is and display this value in a dynamic variable in the email confirmation.

What would be the best way to do this? Are there any quick solutions or plugins? or do I need to custom code it? If so what files need modifying?