This is especially helpful for adding time from the past.

On your iOS device, inside OfficeTime tap the “New Session” button at the top. Instead of pressing the green “Start” button, tap the Start date and time, then change the rollers to the correct date and use either the start/end times, or enter a duration. 

You can also tap the big 0m duration to just change the duration.

Note: If you enter a Start date (which has a time automatically attached) and enter the Duration, it will automatically adjust the End Date/Time to that duration.