Calculating time difference does not deliver the expected result

Dear community,

I need to calculate the time difference between a column named "time_val" (formatted as date/time) as provided in the image below and "1/1/1970 0:00:00". The result should be provided in hours. 

 Input Data

So I created a new column and tried to calculate the time difference with the following expression: 

duration = DATEDIFF(DateValue("1/1/1970"),SomeTable[time_val].[Date],HOUR)

Unfortunately, this only provides the duration in multiples of 24, so it basically seems to calculate the days and multiply the result with 24. What I would need, however, is the exact number of hours.

To give an example: For an entry "01.01.1970 14:20:00" I would expect a response of 14 (hours). What I get, however, is 0.

I'd highly appreciate any help.

Thanks and best regards

Oliver