Sheet: 
(0, 0)		
(0, 1)		DateDiff (custom function; enter as 3-cell horizontal array)
(1, 0)		
(1, 1)		
(1, 2)		
(2, 0)		
(2, 1)		TRUE
(2, 2)		Make the function return an array
(3, 0)		
(3, 1)		TRUE
(3, 2)		Get earlier date
(4, 0)		
(4, 1)		TRUE
(4, 2)		Get later date
(5, 0)		Day1
(5, 1)		25
(5, 2)		Parse both dates into d-m-y
(6, 0)		Month1
(6, 1)		9
(6, 2)		   components
(7, 0)		Year1
(7, 1)		1989
(8, 0)		Day2
(8, 1)		52
(9, 0)		Month2
(9, 1)		18
(10, 0)		Year2
(10, 1)		1991
(11, 0)		
(11, 1)		TRUE
(11, 2)		If we need to carry the day value...
(12, 0)		
(12, 1)		TRUE
(12, 2)		    Add appropriate value to Day2
(13, 0)		
(13, 1)		TRUE
(13, 2)		    And subtract 1 from Month2
(14, 0)		
(14, 1)		TRUE
(15, 0)		
(15, 1)		TRUE
(15, 2)		If we need to carry the month value...
(16, 0)		
(16, 1)		TRUE
(16, 2)		   Add 12 to Month2
(17, 0)		
(17, 1)		TRUE
(17, 2)		   And subtract 1 from Year2
(18, 0)		
(18, 1)		TRUE
(19, 0)		ResultArray
(19, 1)		2
(19, 2)		Get year component of result
(20, 0)		
(20, 1)		9
(20, 2)		Get month component of result
(21, 0)		
(21, 1)		27
(21, 2)		Get day component of result
(22, 0)		
(22, 1)		TRUE
(22, 2)		And return as a horizontal array