Calculated Column Incorrect Calculation

I have 2 columns to track percentages and I would like an overral progress column which has the follwoing SUM

=[Date DFM Started]+[Date GA's Started]

=[30]+[40]

The total column has a value of 89,708 instead of 70. I was trying to get this to work before i worked on dviding the total by 2.

So the average of the 2 columns =([Date DFM Started]+[Date GA's Started])/2

But this did not work? Any help is appreciated.