datetime difference as decimal number

HI all, I need a column to calculate datetime difference between two datetime columns, enddate - startdate. 

Both columns are in this format 

I will add conditional columns which will have as condition the result of this datetime difference compared to 0 (>= or <0 )

So I need the result of the differecen to be expressed in (parts of) hours (or minutes) and also be able to show negative results (start date can be bigger than end date) but when datetimes are very close,  I get 0 instead so my conditional columns i create later are not correct. 

So I need your help please, how can I create a new column for  a difference between 2 datetime columns expressed as decimal number which allows also negative values?  

Thanks!