Yahoo has developed a new online calendar that the company said offers significant improvement over the current product because it makes it easier to share items and has a more interactive interface.

Yahoo will begin to offer the new version of Yahoo Calendar on Wednesday in beta in the U.S., Brazil, India, Taiwan and the U.K. General availability is expected in the coming months.

Although Yahoo Calendar and Yahoo Mail are tightly integrated, only about 8.1 million use the former and about 278 million the latter, said John Kremer, Yahoo Mail vice president.

The situation is similar among other major providers of Webmail and online calendar services, said Matt Cain, a Gartner analyst. Only about 3 to 4 percent of consumers also use their preferred Webmail service's companion online calendar, he said.

There are various reasons for this, including continued use of paper-based calendars at home and of desktop calendar software like Outlook and Lotus Notes in the workplace, Cain said.

However, Gartner sees favorable conditions for significantly boosting the use of online calendars among consumers, from around 4 percent in 2008 to 25 percent in 2012, Cain said.

The factors that will drive up usage include increasing industry adoption of open standards that make online calendar services interoperable, he said. This is coupled with an increasing realization among consumers of the benefits of subscribing to public calendars and sharing their online calendars with friends and family. In addition, younger users are all growing up using online calendars, he said.

"There's no question that online calendars will emerge as a very important part of portal collaboration software offerings for Yahoo, Microsoft, Google and AOL," Cain said.

"For anyone with a big e-mail population, there's no doubt that the next big battleground will be at the calendar level," Cain said, adding that online calendars will become prime real estate for advertising.

Yahoo is hoping that the new Yahoo Calendar will prompt more Yahoo Mail subscribers to use it. Yahoo Calendar hasn't gotten a facelift of this magnitude in about 10 years, Kremer said.

Among the features Yahoo is highlighting in the new calendar service is its compatibility with competing products from providers like Mozilla, Apple, Microsoft, AOL and Google.

This compatibility, which will allow Yahoo users to share calendar data with users of those other services, is possible because Yahoo Calendar is built on open standards like iCalendar (iCal) and CalDAV, Kremer said.

Yahoo hopes that open calendar standards are adopted broadly not only among online service providers but also among makers of business desktop and mobile calendar software, so that Yahoo Calendar will become interoperable with those products, Kremer said.