Return a blank cell with no entry

[Kidmon]

I am using excel 2007 & 2010

I have 2 problems:

1) I need to change a formula that now calculates the military time difference between column J and column K. This formula works very well but I need it to also not calculate when only coulmn K contains a date.

When both column dates are entered it is Ok, when only 1 column date is entered I need column L to return a blank cell not a value of K.

2) How can I use coulmn M to count down 30 min from the time entered in column J ?
Thanks again, forum contributors a very appreciated.
K

Excel Workbook
	J	K	L	M
5
6	Time Out	Time In	Total Time Out	Time Left
7
8
9	1110	1113	3
10	1123	1129	6
11	1111	1234	123
12		1115	1115
DDAR Form

Excel 2010

Cell Formulas
Range	Formula
L9	=IF(K9-J9=0,"",K9-J9)
L10	=IF(K10-J10=0,"",K10-J10)
L11	=IF(K11-J11=0,"",K11-J11)
L12	=IF(K12-J12=0,"",K12-J12)

 

[Robert Mika]

Your first question can go like this:=IF(COUNTA($J9:$K9)=1,"",K9-J9) What would you expect in column M? 1110-30?

 

[Kidmon]

Problem 1:
Thank you your solution for problem 1 works great. BUT it revealed a new problem. If BOTH column J and column K are empty, I need column L to return blank. Your formula works great but it returns "0" until I enter a date in column K. How can I make column L return "0" if both J & K are empty?

Problem 2: How to create a "Count down timer". Do I want column M to count to 30 is YES. You assume correctly.
Thank you again.