Excel 2010: IF condition and #VALUE

Hoping someone can help me with this as I've been trying to wrap my brain around this all morning.

I have the following formula in my spreadsheet:

=IF(T6="SLP",N6-3, IF(T6="La Grange", N6-13, IF(T6="Monclova", N6-3, IF(T6="Hodgkins", N6-13))))

in some cases the value in column N are 00/00/0000, which results in my formula spitting out an #VALUE...
The formula works fine everywhere else...

How can I get it to display a 0 or simply have it blank instead?

Thanks
David