|I did say I might not have explained it well, that's why I included the list to illustrate the required answers.|
Assuming the first date is 1st Sept then the next answer would be 15th. You could take any date *in that list* to begin but the next answer would be the following date.
If today was anywhere between 1 Sept & 14 Sept, answer would be 15th. If today was anywhere between 16 Sept & 28th Sept answer would be 29th and so on.
I know I could simply use a list by adding 14 to the previous date but that wouldn't allow for a "today" in between those listed dates.