It plans to use the cash from the sale to fight off stiff competition and stay ahead as the world's most used search engine.
Google Calendar is part of its expansion into new areas, pitting the search giant in direct competition against Yahoo.
Yahoo has offered a calendar service since 1998 and in October bought the event planning site, Upcoming.org.
Yahoo is currently the most popular web calendar service in the US.
In response to Google's announcement, Yahoo said it would release updates to its calendar service in coming months.
Language processing
Google's service is designed to be easy to use, said Carl Sjogreen, product manager for Google Calendar.
As well as being integrated with Gmail, the calendar uses so-called natural language processing technology to simplify how events are added.
It means that people can type an entry like "lunch on Sunday 12pm" and the software will automatically add it to the calendar.
"We enable the user to create multiple calendars, share them with other people and overlay web calendars back on the user's own calendar,"said Google Calendar product manager Carl Sjogreen.
At the moment, the calendar works best with Microsoft's Internet Explorer and Firefox. In the coming months, Google aims to make it synchronise with Outlook and mobile devices.
