excel formula, if, and, or?

I am making an Excel form to use in housing. I want to be able to enter a date in one cell (tenancy end date) and calculate the number of days empty in the next cell (void days). I then have another column with date re-let.

I have managed to get it so that the void days column will stop calculating when a date is entered in the re-let column. Every week, I have to report on the number of void days for all the properties - this will be easy if I have a tenancy end date on each line, however, I don't. This means that when I copy the formula =IF(ISBLANK(J6),TODAY()-H6) onto the next line, excel inputs 42995 - not good when I want to add up that column.

I found a formula =IF(OR(ISBLANK(J11),ISBLANK(H11)),0,TODAY()-H11) to put a zero in the void days column, HOWEVER, I need it to calculate the number of void days still if there is a date in the tenancy ended column BUT, to stop calculating the number of void days when a date is entered in re-let date. I use excel by googling a lot, so, I am not an expert, just a good googler!