Median difference between two dates

Hi, Thanks in advance for taking a look!

I am trying to calculate the median difference between two dates but I am running into trouble. Here is sample data:

A B
1/1/13 2/1/13
1/2/13 2/5/13
1/4/13 1/25/13
1/7/13
1/10/13 1/18/13


I want to be able to type notes into column B such as "closed" or simply "X" (so i can keep track):

A B
1/1/13 2/1/13
1/2/13 2/5/13
1/4/13 1/25/13
1/7/13 X
1/10/13 1/18/13


I tried this:
in column c, use "datedif" function then calculate median from column C values.
problem: if left blank, value in row 4 comes up as #NUM!. if text is entered, value comes up as #VALUE!

I also tried this array: =median(abs(B5:B1-A5:A1))

Should i be trying the medianif function?

Thank you for your help, any feedback is appreciated!!

figured it out, but can't delete this post.

i used an array with column c as days between
=median(if(isnumber(c1:c5),c1:c5))