Specified cast is not valid (double precision to int)

What you're doing is equivalent to:

object x = 32.5;
int y = (int) x;

You can't do that - when you unbox, you have to unbox to the actual type of the value¹.

So you'd need:

object x = 32.5;
int y = (int) (double) x;

The cast to double unboxes, and the cast to int converts the double to an int. You can do the same in your code:

(Int32)(Double) reader["DateDiff"] : 0;

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¹ Well, modulo a few things. The CLR allows you to unbox an int to a uint or to an enum type with an underlying type of int, etc.