Microsoft has extended the testing period for the Windows 7 beta after admitting that it had fumbled the expiration date.

In an email, Microsoft apologised for the error. It said that rather than commencing periodic shutdowns from next week, Windows 7 beta will do so from 1 July.

A newsletter email from Microsoft HQ headed 'Exploring Windows 7 - Correction' was sent out to beta users today outlining the dates involved.

The body text of the email message reads: "Because you signed up to test the Windows 7 Beta, we recently sent you mail about the expiration dates for the Beta and Release Candidate. Unfortunately, we made a mistake.

"We said the Beta would start shutting down every two hours on June 1, 2009. The correct date is July 1, 2009."

The auto-shutdowns start date for the Windows 7 Release Candidate remains as originally stated - 1 March 2010.

Although information published on the web and elsewhere has made it clear that the beta of Windows 7 expires fully on 1 August with shutdowns happening without warning throughout July, Microsoft seems not to have given those who downloaded the beta software the same information.

Microsoft managed to confused some beta testers by issuing an email last weekend warning them that their Windows 7 beta trial was close to expiration. Yesterday, Microsoft blogger Brandon LeBlanc issued a clarification on the Windows 7 blog while today beta testers have received a correction email.

The Windows 7 beta code was offered as a freely available download for a two-month period from January this year.

At the end of April, the RC1 version of Windows 7 (the feature-complete version of the operating system) went live. This can be downloaded from Microsoft's Windows 7 site. Users will be able to make full use of it until next year.

Microsoft won't yet commit to a specific shipping date for its new operating system but is widely expected to launch Windows 7 commercially this autumn.