Answer:
nth day: w:x:(y-2(n-1))
Explanation:
Let Original time = w:x:y
Where w is the hours, and x is the minutes, and y is the seconds.
Sadie's time on the first date: w:x:(y-2)
Second day: w:x:(y-4)
Third day: w:x:(y-6)
and so on
nth day: w:x:(y-2(n-1))