|from two different resources|
Most likely either one or both your Dates are not DATES as Excel sees them,
but Text Strings that look like Dates.
A quick way to determine if they are real DATES,
select both date cells and format them as Numbers.
If they still display as Dates, IE 12/31/2013 then they are TEXT Strings.
If they display as a four digit number like: 42004 then they are real DATES.