Help with IF and Isblank function

I need help with the following formula. =IF(ISBLANK(U4)&(V4),(""),(IF(ISBLANK(V4),(T4-U4),(T4-V4)))). I almost have it correct but it is not giving me the end result I am looking for. I am working with dates. Column T is the first estimated day a part would be repaired. Column U is an extension to the first estimated day. Column V is the actual day the part arrived at my shop. The end result I am looking for is the time between the first estimated date and the delivery date. I would like for the function to tell me the time between the fist estimated date and the new updated date when the part has not arrived in my shop yet. If I only have the first estimated date entered and nothing else then the cell of days should be blank. My first attempted formula was =IF(ISBLANK(V4),(T4-U4),(T4-V4)). This gave me the right numbers except in the cells where I only have the first estimated date kept giving me a 40000 number instead of being blank. I hope this didn’t confuse you guys.

Orginal Date New Date Actual Date Days
11/19/2010 11/16/2010 #VALUE!
3/18/2011 11/18/2011 -245
1/21/2011 2/14/2011 #VALUE!
2/4/2011 3/10/2011 #VALUE!
2/4/2011
2/20/2011
5/12/2010 10/12/2010 8/12/2010 #VALUE!

They're not actually blank - they have a value of zero:

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In W4:

=IF(AND(ISBLANK(U4),ISBLANK(V4)),"",(IF(ISBLANK(V4),(T4-U4),(T4-V4))))