Microsoft worked to patch the first reported bug in Windows 98, which throws the operating system's calendar off by one or two days if it is rebooted in the seconds before the date rollover.
Last week, when the bug first was reported by a British Internet company, Microsoft said the flaw only affected systems if they were rebooted at the precise moment when a new year date is rolling over, on Dec. 31.
However, on Monday the company said the bug hits on any day, causing the Windows 98 calendar to jump forward two days, or back one day.
"It can happen any night," a Microsoft representative said.
Microsoft worked on a fix Monday and planned to post it to the company's Windows Update Web site, the representative said.
Last week, Windows product manager Kim Akers rated the probability of the bug happening at "one in six million," and said the length of the rollover window varies, depending on the speed of the PC.
The flaw was discovered by Bromley Computer Systems, a Shropshire, England, company that was using Prove It 2000 software to test for millennium compatibility.
Although the date rollover bug was the first confirmed bug in Windows 98, many users have reported installation and upgrade problems since the operating system was released June 25.
A package of updates to Windows 98, mostly multimedia enhancements, now is in beta testing. It is unclear whether the package will include the date rollover bug fix.