when we press F1 on "Choose company " screen there is a note under "Logging on to companies" with result title..
"Once your are logged on to a company, the company name and user name are displayed at the top of the SAP Business One Main Menu. The current posting period date appears in square brackets next to the user name, for example:
Sales Manager [March 2007]"
But when we do login in SAP there is only user name but not date as mentioned in Help.