Hi,
Recently we added support for posting future publish dates when using Live Writer. I'm guessing this must have introduced this error if you don't specify a publishing date. I will fix this for the next release. In the menatime you can either specify a publish date from LiveWriter or add this to you user.config
<add key="DisableUseOfPassedInDateForMetaWeblogApi" value="true" />
and then it will just use the current time for publish date.
Please let me know if these workarounds solve it for you.
Thanks,
Joe