Modification a Script for Top 10 Senders

Sorry Guys, but i got the following error:

Cannot convert value "20/03/2013" to type "System.DateTime". Error: "String was not recognized as a valid DateTime."
At C:\Users\cagutpen.admin.net\Desktop\Exchange2010EmailStats.ps1:9 char:36
+ $outfile_date = ([datetime]$rundate <<<< ).tostring("yyyy_MM_dd")
    + CategoryInfo          : NotSpecified: (:) [], RuntimeException
    + FullyQualifiedErrorId : RuntimeException

I run then the Get-Date and i got the next:

[PS] C:\Users\cagutpen.admin.net\Desktop>Get-Date

jueves, 21 de marzo de 2013 02:36:37 p.m.

What I need to change on the script to work for me???

Thanks for any advice