Calculating dates based on a particular cell

I am having problems working this one out even though it seems like it would be pretty simple!

I need to work out the formula for calculating the dates based on a particulr cell. Ie, when I put a date in one cell, I need other cells to calculate the date say 7 days, 14 days, 50 days etc before or after that particular point.

Can anyone point me in the right direction?

Oh my god i feel so stupid!!