I am trying to check if a certain number of days has passed since a date and if it has then change the color of the grid row.
So if the date is 12/11/2016 and I want to check if 10 days has passed since that date.
if (dt.Date > dt.Date.AddDays(10))
Did you mean to use today's date? If so, that's
if (DateTime.Now > dt.Date.AddDays(10))
At the moment you are comparing
dt.Date against same date plus 10 days - as others noted, this will never be met.
Update. As Tim suggests in comments, using
DateTime.Today might be more appropriate.