I have similar SO question for date calculation for "Bussines days".
And finaly I use mechanism described here.
But this is the particular case.
What about more general approach?
How add specific number (n) of days to current date considering only the days from custom set: Mn-Tu-We or Sa-Su-Me or any other.
Test data 1:
Date: (28-jun-2014).
Days to add: 10
Days type: Mn - Th - Fr
Answer: 21-july-2014
Test data 2:
Date: (28-jun-2014).
Days to add: 5
Days type: Tu - We
Answer: 15-july-2014