Time field is filled with "0:00:00" problem

Dear,

I have three fields "Actual Hours","Actual Start","Actual End".User can fill "Actual Hours" or "Actual Start","Actual End" and calculate "Actual Hours" by "Actual Start" and "Actual End".

If the  "Actual Start" field is filled with 0:00:00,  if using the actual end time to minus the actual start time, the result should be 16, but the result is empty. 

Quick feedback will be appreciated!

Best regards,

Hu Hongfeng

Tags edited by: Michael Appleby