This might also work for you
omnirpt -report session_statistics -timeframe [start date] [finish date] -log [filename]
The timeframe can be stated either in absolute terms, for example
omnirpt -report session_statistics -timeframe 2014/01/21 00:01 2014/01/27 23:59 -log C:\week_4_stats.txt
or in relative terms based on the time the command is issued
omnirpt -report session_statistics -timeframe 72 72 -log C:\3_days.txt
You can add an argument '-tab' to seperate the fields with 'tab' characters, which will preent truncating of a field, and make the report suitable for import into Excel
omnirpt -report session_statistics -timeframe 72 72 -tab -log C:\3_days.txt
or, if you want the output file in a nice HTML format
omnirpt -report session_statistics -timeframe 72 72 -html -log C:\3_days.htm
There are also filters to include only certain datalists, or to exclude what you don't want. All of this can be found in the CLI (Command Line Interpreter) Guide in the GUI by clicking 'Help -> Guides'