Date calculations

[JoseGarcia]

Hi there, 

need to substract date & time dimension to give me total time taken.
Dimension data is displayed: 

formula is:
axle1finish - axle1start

Using date & time function, but unable to get data in a way that allows me to perform any calculation.
Could anyone assist?
Thanks

[JoseGarcia]

@stevejoyce & @mato32188 

Thank you both! 

Time stamping & using interval function worked.