You don't need to do things like add or subtract in order to calculate flight time. Just enter the departure and landing time and the program will calculate it for you and also you can calculate Duty time by entering OFF Block and ON Block. Still you may calculate multiple times on Multiple Time Calculator page.

This application was made to make it easier for pilots to enter their flights into logbooks.

It has been made according to 24-hour time frame.

The value displayed as the result indicates the time spent in the air.
NOTE : GDP and DP results may change by country.