Returning logs after a specific date?

Hi Guys, 

I am trying to find Errors in the system log. Only logged after the 1st of July 2011. However my command doesn't return anything. 

Is the format of my date wrong? 

Get-eventlog system -entrytype error | Where-Object {$_.Time -ge [datetime]"01/07/2011 9:00 AM"}

Thanks.