#value error

I need to make this formula

if g3 and j3 are blank then h3 is blank.

if g3 has a date and j3 is blank then h3=g3-b1.

if g3 has a date and j3 has a date then h3="Done"

=if(and(isblank(j3),isblank(g3)),"",if(and(not(isblank(g3)),isblank(j3)),g3-b1,"Done"))

this is my formula but I don't understand the problem! 

Hope someone can help, many thanks!