The string was not recognized as a valid DateTime. There is a unknown word starting at index 0.
To start with, it looks like one of the code lines like
is throwing the same. Based on error, the value is either empty or not in a format that can be converted into a Datetime.
Make sure, you have a valid datetime value for conversion.
Moving ahead, the filter criteria formed should be proper as expected. Use VS DEBUGGER to track and see the workflow and values.