New Post: ParseExact Exception

I'm receiving it after running the following command...

$certLookup = Get-CertificationAuthority | Get-IssuedRequest -filter "CommonName -eq $cert"

$cert would be a FQDN such as server1.contoso.com
$certLookup would contain entries such as...

RequestID : 23409
Request.RequesterName : Domain\Requestor1
CommonName : server1.contoso.com
NotBefore : 7/29/2013 5:42:04 PM
NotAfter : 7/29/2014 5:42:04 PM
SerialNumber : 51bf92eb000012305b71
ConfigString : PKIserver.contoso.com\Contoso Root CA

RequestID : 24839
Request.RequesterName : Domain\Requestor2
CommonName : server1.contoso.com
NotBefore : 8/13/2013 4:29:43 PM
NotAfter : 8/13/2015 4:29:43 PM
SerialNumber : 8dbca20d00012306107
ConfigString : PKIserver.contoso.com\Contoso Root CA