A small sample application that can be reloaded would definitely help.
To match your values, you can consider rounding your timestamps (apply the same rounding to both field values):
Round( [Signal Date], Maketime(0,15) ) as RoundedSignalDate,
Round( [Pickup Date], Maketime(0,15) ) as RoundedPickupDate,
(maybe you want to use other rounding functions like Ceil() or Floor())
You may also want to consider using a canonical date: