How to calculate Time deffrence?

Hi

i designing report in query designer :

Start date time : 20,090,610,063,800.000

Completing date time : 20,090,610,063,854.000

I have 2 questions

1. how can i extract time deffrence between them ( because i need to generate records in leas than 48hrs)

2. How can i get time and date seperated when it is in this format : 20,090,610,063,800.000

Babu