Policy Based Management: System Failure Error Check Policy - Filter by TimeGenerated?

I have been searching for a means to change the System Failure Error Check policy that comes as part of the Best Practice policies. I want to look back 24 hours. The WQL query shipped with the policy doesn't have a WHERE clause component that looks at TimeGenerated. That query looks like:

IsNull(ExecuteWql('Numeric', 'root\CIMV2', 'select EventCode from Win32_NTLogEvent where EventCode=6008 and Logfile="System"'), 0)

After searching for an example of how to do this and not finding any that are specific to PBM, I decided to fall back to a very basic approach - use wbemtest.exe to try out where clause additions and see how they work, then plug the result into the policy and see if it works. As a start, I tried the following query using wbemtest.exe:

select EventCode
from   Win32_NTLogEvent
where  EventCode = 6008
and    Logfile = 'System'
and    TimeGenerated > '20130101010000.000000–000'

This works great in wbemtest.exe. My next step was to plug this into the policy condition expression as follows:

IsNull(ExecuteWql('Numeric', 'root\CIMV2', 'select EventCode from Win32_NTLogEvent where EventCode=6008 and Logfile="System" and TimeGenerated > "20130101010000.000000–000"'), 0)

When I try to manually evaluate this policy in SSMS, I receive an "Invalid Query" error message. I am highly puzzled. I have seen the Scripting Guys post - I assume that SWbemDateTime isn't available to use inside Policy Based Management policies. All the examples of how to handle the kind of dynamic date creation I have seen are for use in PowerShell, VBScript, or SSIS. I've played with using DateDiff, DateAdd, and GetDate inside the query string, with no success.

So here are my questions:

1. Why does the ExecuteWql above fail?
2. Is it at all possible to dynamically generate a datetime (say, 24 hours ago) as part of the query string parameter of the ExecuteWql call?
3. What might that look like?

I appreciate any thoughts, feedback, or pointers!