Currency Conversion

Dear All

How does system Calculate the Currency Conversion in SAP Like INR TO USD.How do we update the currency conversion day to day wise do we have any standard tool to automatic conversion based on daily currency rate from Internet.

Suppose if i am having Sales office in India plant in abroad. System what time will generate the invoice Indian time or Abroad Time .What are the setting we need to do in the system to capture the abroad Time.

Regards,