How to filter CDO export on Date Modified using Bulk API

I'm using the Bulk API 1.0 and E10, and I'm trying to export custom data object (aka CDO, Data Card) records that have been modified recently. I can get it to work using another date field, but I can't figure out what the field name or ID is for the modified date. I've tried C_DateModified, but that returned an generic error message. I tried to query the custom object to get all fields, but that just returns the custom fields. Any ideas?

By the way, the filter I'm using is something like this: filter":{"filterRule":"valueGreaterThanOrEqualToComparisonValue","value":"{{CustomObject[53].Field[???]}}","comparisonValue":"2014-02-01 00:00:00"}

Thanks!